The 3Rs

Hamsters - Photo by Michele Cunneen

In 1959, a landmark book, The Principles of Humane Experimental Technique, was published by William Russell, a zoologist, and Rex Burch, a microbiologist. As a foundation for the principles set forth in the book, the authors assert that, “it is widely recognized that the humanest possible treatment of experimental animals, far from being an obstacle, is actually a prerequisite for successful animal experiments.” The book broke new ground by introducing the concept of the “3Rs”—replacement, reduction and refinement—to the treatment of animals in research:

  • Replacement refers to achieving a research goal or objective without conducting experiments on animals. Methods are employed whereby animals are replaced with suitable alternatives.
  • Reduction refers to the use of methods that allow researchers to obtain comparable levels of information using fewer animals.
  • Refinement refers to practices that reduce or eliminate the animals’ pain, stress and discomfort—not only during experimental procedures, but in relation to the animals’ daily social and physical environments, as well.

Around the world, among those who accept the tenant that humane treatment is a prerequisite to successful experiments—as well as a moral obligation—the 3Rs have been embraced as guiding principles for the conduct of research, testing and teaching.

The UK’s Universities Federation for Animal Welfare funded the work conducted over five years by Russell and Burch that ultimately led to publication of the book. AWI’s founder, Christine Stevens, provided helpful recommendations to the authors and is acknowledged in the preface. In addition, AWI provided funds to make possible a later reprint.

The full text of the book can be found on the website of the Johns Hopkins Bloomberg School of Public Health

Citation: Russell, W.M.S. and Burch, R. L. (1959). Charles C. Thomas Publishing, Springfield.


“Replacement” is one of the alternatives to animals in research know as the 3Rs as described in The Principles of Humane Experimental Technique. Authors Russell and Burch define a replacement technique as “any scientific method employing non-sentient material which may in the history of experimentation replace methods which use conscious living vertebrates.” Scientists should seek to replace animals in experiments with alternatives whenever possible.

Replacement alternatives include the use of human volunteers, in vitro systems, mathematical and computer models and simulations, microorganisms, bacteria, plants and fungi, and imaging techniques. Some replacements can be done completely without use of other living beings—computer models and simulations for example. At present, however, there is a far greater reliance on in vitro replacements that rely on cells or tissues taken from donor animals, as well as from humans and early-development human or animal embryos. A single donor may provide tissue that can be used for a large number of cultures and alleviate the need for multiple test subjects.

Partial replacement is also possible. For example, alternative methods might be used to pre-screen substances for toxicity. If preliminary in vitro tests show the substance is toxic, there is no need to test on animals. Only a negative result would lead to additional testing on animals.

A thorough review of the scientific literature can help researchers find replacements—as it may indicate that the work has been done previously or that there are other scientifically valid means to obtain the desired data.


“Reduction” is one of the 3Rs alternatives to animals in research as described in The Principles of Humane Experimental Technique. Authors Russell and Burch define reduction in this context as “reduction in the numbers of animals used to obtain information of given amount and precision.” In other words, scientists should seek to reduce the number of animals in experiments to the minimum required to obtain scientifically valid data. Reduction also can be achieved by obtaining more information from a given number of animals—so as to reduce the need for additional animal subjects.

Researchers are continuously increasing their understanding of effective experimental design and statistical methods. An increased awareness of the ethical reasons for reducing animal use is also on the rise. From a practical standpoint, the high cost of animal subjects provides further incentive to minimize the number of animals in research.

There are a variety of ways that scientists can reduce the numbers of animals in research. Russell and Burch note that, “One general way in which great reduction may occur is by the right choice of strategies in the planning and performance of whole lines of research.” For example, a small pilot study, using a small number of animals, may indicate to researchers whether a larger study is appropriate.

In some cases, in vitro (test tube) experiments can be used to screen appropriate experimental design, and may indicate the feasibility of a larger study or ways a study could be modified to use fewer animals or less invasive procedures. Maximization of the data from each procedure, reduction of data skewing extraneous variables, data sharing to avoid repetition, harmonization of regulatory requirements for testing, and the use of computer-assisted teaching of experimental design and statistics may also streamline experiments and alleviate the need for more animals.

The field of bioinformatics can also help reduce the need for animal subjects. Bioinformatics is an emerging, interdisciplinary field that draws on computer science, mathematics, and information theory to model and analyze biological systems, especially systems involving genetic material. Bioinformatics can be used to analyze complex experimental results from multiple sources, patient statistics, and scientific literature. This amalgamation of biomedicine and computer technology can permit the gleaning of pertinent information from both past and ongoing experiments—allowing scientists better access to, as well as analytical tools to interpret, data already gathered.


Suppose, for a particular purpose, we cannot use replacing techniques. Suppose it is agreed that we shall be using every device of theory and practice to reduce to a minimum the number of animals we have to employ. It is at this point that refinement starts, and its object is simply to reduce to an absolute minimum the amount of distress imposed on those animals that are still used.

—W.M.S. Russell and R. L. Burch
The Principles of Humane Experimental Technique

Every researcher subjecting animals to research, testing or teaching has the responsibility to implement refinement alternatives, one of the three Rs described by Russell and Burch. The goal of refinement is to minimize distress in animals in research.

A wide array of refinements can be implemented. Some involve improvements in living quarters: Social animals might be provided with compatible companions, or caging modified to permit and encourage species-typical behaviors (e.g., foraging, climbing, grooming, and burrowing). Others involve less stressful handling, such as when animals are trained to cooperate for procedures rather than being subjected to enforced restraints (a mouse trained to voluntarily drink from a syringe and thereby ingest a particular substance, or a monkey induced through positive reinforcement to offer a limb for a blood draw in exchange for a reward).

Other forms of refinement include:

  • Use of non-invasive imagine techniques such as magnetic resonance imaging (MRI) and computed tomography (CT);
  • Use of the anesthetic, analgesic and tranquilizing drugs in the proper dose and administration to effectively alleviate pain or distress;
  • The development and use of “humane endpoints”—defined by one source as “the earliest indicator in an animal experiment of (potential) pain and/or distress that, within the context of moral justification and scientific endpoints to be met, can be used to avoid or limit pain and/or distress by taking actions such as humane killing or terminating or alleviating the pain and distress.”