Interferon alfa-2a (Roferon-A, Hoffmann-La Roche Inc., Nutley, NJ) is identical to one of approximately 15 subtypes of interferon alpha made by human leukocytes and is produced in bacteria using recombinant DNA techniques. In its antiviral, antiproliferative, and immunomodulatory activities it is similar to leukocyte interferon alpha. These activities are species-restricted and have been demonstrable, thus far, only in humans, certain other primates, bovines, and guinea pigs or cells derived therefrom. The possibility that the toxicity of interferon alfa-2a would also be species-restricted appears to have been confirmed by results obtained thus far. Toxicological studies in rats, mice and several species of monkeys have failed to indicate the side effects that have been observed in humans. However, studies in species in which interferon alfa-2a is active and in others in which it is not, have revealed similar pharmacokinetics and elimination mechanisms.