Oral administration of BL-3849A to adult mice resulted in peak serum interferon titers of 4,000 units from 15 to 30 h after administration, with detectable levels persisting until 48 h. After intraperitoneal (i.p.) inoculation, peak serum interferon titers of 1,000 to 3,000 units were noted between 9 and 18 h. Multiple injections of the inducer by either route resulted in a marked decrease in the interferon response with each successive dose. In mice infected intranasally with the Rochester mouse virus strain of encephalomyocarditis virus, oral treatment with BL-3849A reduced mortality when initiated either 18 h before or 1 h after infection. In contrast, administration of drug by the i.p. route decreased mortality only if begun before infection. In mice inoculated i.p. with encephalomyocarditis virus, treatment by both the oral and the i.p. route decreased the mortality whether initiated 18 h before or 1 h after infection. Treatment by the oral, but not the i.p., route reduced mortality of mice inoculated i.p. with Semliki forest virus or Herpesvirus hominis type 2. BL-3849A appeared to be as effective as tilorone hydrochloride, but less effective than polyriboinosinic-polyribocytidylic acid, in the treatment of these viral infections of mice.