The driving force behind the explosion of in vitro science in the last 10-15 years has been the concern for animal welfare. The concept of reduction, replacement, and refinement was conceived and developed by W. M. S. Russel and R. L. Burch (1959) in their book The Principles of Humane Experimental Technique. Reduction means reducing the numbers of animals used without compromising results. Reduction lowers the number of animals involved by intelligent consideration of the known facts, including in vitro test results. It is a principle well followed in practice. For instance, Procter & Gamble reports a reduction of over 90% in the number of animals they use. Replacement refers to substitution, either wholly or in part, of the animal model with a nonanimal model. This includes substitution with knowledge bases and in vitro tests or sometimes with an animal lower on the taxonomic scale. Refinement includes techniques that can reduce or eliminate pain in the animal model. Examples are use of the low-volume Draize eye irritation tests and changing the LD50 to measure onset of toxicity rather than ani-

mal death, as well as limiting dose level to eliminate sheer volume effects.