Viramidine, the 3-carboxamidine derivative of ribavirin, was effective against a spectrum of influenza A (H1N1, H3N2 and H5N1) and B viruses in vitro, with the 50% effective concentration (EC50) ranging from 2 to 32 microg/ml. The mean 50% cytotoxic concentration (CC50) in the MDCK cells used in these experiments was 760 microg/ml. Ribavirin, run in parallel, had a similar antiviral spectrum, with EC50 values ranging from 0.6 to 5.5 microg/ml; the mean CC50 for ribavirin was 560 microg/ml. Oral gavage administrations of viramidine or ribavirin to mice infected with influenza A/NWS/33 (H1N1), A/Victoria/3/75 (H3N2), B/Hong Kong/5/72 or B/Sichuan/379/99 viruses were highly effective in preventing death, lessening decline in arterial oxygen saturation, inhibition of lung consolidation and reducing lung virus titers. The minimum effective dose of viramidine in these studies ranged from 15 to 31 mg/kg/day, depending upon the virus infection, when administered twice daily for 5 days beginning 4 h pre-virus exposure. The LD50 of the compound was 610 mg/kg/day. Ribavirin's minimum effective dose varied between 18 and 37.5 mg/kg/day with the LD50 determined to be 220 mg/kg/day. Viramidine's efficacy was also seen against an influenza A/NWS/33 (H1N1) virus infection in mice, when the compound was administered in the drinking water, the minimum effective dose being 100 mg/kg/day. Delay of the initiation of either viramidine or ribavirin therapy, using the approximate 1/3 LD50 dose of each, was protective as late as 48 h after exposure to the A/NWS/33 virus. While both compounds appear to have similar efficacy against influenza virus infections, when one considers the lesser toxicity, viramidine may warrant further evaluation as a possible therapy for influenza.