A total of 200 influenza A virus strains were studied. Among them there 32 strains of A(H1N1) and 84 strains of A(H3N2) from the 2002-2003 epidemic season and 84 strains of A(H3N2) from the 2003-2004 epidemic season. Most rimantadine-sensitive strains whose infectivity and hemagglutinating activity were found to decrease by the drug given at a concentration of 0.5 microg/ml. In terms of infectivity and hemagglutinating activity, the number of rimantadine-resistant strains of A (H1N1) was equal to 9.0% for A(H1N1) influenza viruses and to 10.0 and 14.0% for A (H3N2) influenza viruses in different seasons, respectively.