Because medical countermeasures to potential bioterrorism threats cannot be evaluated for efficacy in humans by traditional human clinical trials, the Food and Drug Administration “Animal Rule” allows for approval of such countermeasures by demonstration of efficacy in animals which must then be extrapolated to predict efficacy in humans. The challenge posed by the Animal Rule is to establish an approach whereby animal efficacy data from multiple species is appropriately analyzed so that a constructive prediction of human efficacy can be made. This presentation will offer considerations for the application of the Animal Rule for anthrax vaccines. Examples from multiple challenge studies in multiple species will be presented.