The purpose of this guidance is to assist sponsors in the clinical development of drugs for treatment or prevention of smallpox (variola virus) infection. Clinical efficacy trials of drugs for treating or preventing smallpox are not feasible and challenge studies in healthy subjects are unethical; therefore, drugs for these indications should be developed and approved under the regulations commonly referred to as the Animal Rule (21 CFR part 314, subpart I, for drugs and 21 CFR part 601, subpart H, for biologics). This guidance serves as a focus for continued discussions among the Division of Antiviral Products (DAVP), pharmaceutical sponsors, the academic community, and the public.