Five scenarios are proposed that investigators and clinicians might consider in the evaluation of an investigational drug for postexposure chemo-prophylaxis. They are (a) safety as the only standard required for an indication for postexposure chemoprophylaxis; (b) efficacy of the drug for treatment of HIV-infected patients as a sufficient criterion for an indication for postexposure chemoprophylaxis; (c) a classic placebo-controlled trial as the basis for evaluation of postexposure chemoprophylaxis; (d) a clinical trial design that evaluates outcome in relation to varying times from exposure to initiation of treatment; and (e) combination therapy. Federal regulations are sufficiently flexible to allow demonstration of safety and efficacy of an investigational drug for this indication. The necessary element is that both safety and efficacy be demonstrated in an objective and reliable manner.