The influenza virus neuraminidase inhibitor RWJ-270201 (cyclopentane carboxylic acid, 3-[cis-1-(acetylamino)-2-ethylbutyl]-4[(aminoiminomethyl)amino]-2-hydroxy-[cis, 2S, 3R, 4R]) was significantly inhibitory to an infection in mice induced by influenza A/NWS/33 (H1N1) virus when oral gavage (p.o.) treatment with 10 mg/kg per day was delayed at least 60 h after virus exposure. Treatment was 5 mg/kg twice daily for 5 days. Viral challenge doses of influenza A/Shangdong/09/93 (H3N2) virus ranging from the LD70 to the LD100 did not affect the marked antiviral efficacy of 12.5 mg/kg of RWJ-270201 administered p.o. twice daily for 5 days beginning 4 h pre-virus exposure; infection by an approximate 2 LD100 dose (108 cell culture infectious doses/ml) was only weakly inhibited by the same treatment as seen by significant increase in mean day to death. Murine infections induced by influenza A/Bayern/57/93 (H1N1) and B/Lee/40 viruses were significantly inhibited by 100, 10, and 1 mg/kg per day of RWJ-270201 using the above treatment regimen; influenza A/PR/8/34 (H1N1) virus infections in mice were only moderately inhibited, the antiviral effects using this virus being lessening of arterial oxygen decline, reduced lung consolidation, and inhibition of lung virus titers primarily at the higher dosages.