The monoclonal antibody designated C179 was found to neutralize all of the H1 and H2 strains of influenza A virus studied (Y. Okuno, Y. Isegawa, F. Sasao, and S. Ueda, J. Virol. 67:2552-2558, 1993). In the present study, the ability of C179 to protect mice from the lethal effect of the A/FM/1/47 (H1N1) strain was examined. When the mice were injected intraperitoneally with 100 micrograms of C179 per mouse a day before the virus challenge (2.0 x 10(3) focus-forming units per mouse), all of the mice survived. Moreover, significantly higher survival rates were observed in mice receiving 1,000 micrograms of C179 per mouse 2 days after the virus challenge than in those receiving phosphate-buffered saline alone. These results indicate that C179 is effective not only for prevention but also for treatment of mice infected with H1 and H2 strains. The possibility that C179 can be used for passive immunization in humans is discussed.