The efficacy of bovine interferon alpha C as an antiviral compound was evaluated in calves infected with vaccinia virus. Calves treated with the interferon did not develop characteristic vaccinia lesions, demonstrating a clear protective effect of treatment. The only side effect observed was hyperthermia. After intramuscular injection, interferon appeared quickly in the blood and was cleared within 24 hours. The in vivo antiviral activity of bovine interferon alpha C was confirmed by its ability to induce 2'5' A synthetase, an enzyme implicated in the antiviral state, in lymphocytes.