The subgroup characteristics of 71 strains of respiratory syncytial virus (RSV) isolated in Sapporo, Japan, during 5 epidemic years from 1980 to 1987 were determined by the use of 17 monoclonal antibodies (MAbs) raised against the RSV Long strain, which is now recognized as the prototype subgroup A strain. Nine of these MAbs immunoprecipitated the fusion protein (F), five immunoprecipitated the large glycoprotein (G), two immunoprecipitated the nucleoprotein (NP), and one immunoprecipitated the phosphoprotein (P). Based on the pattern of reaction of these MAbs to RSV isolates in an indirect immunofluorescence assay, we were able to distinguish two different subgroups. Subgroup A strains reacted to all 17 MAbs. Subgroup B strains reacted to none of the anti-G MAbs, eight of the nine anti-F MAbs, and all anti-NP and anti-P MAbs. Subgroup A included 38 (53.5%) isolates from every epidemic year. Subgroup B included 32 (45.1%) strains isolated in the last 4 epidemic years. One virus strain with an intermediate character of reactivity was isolated in 1983. From the first epidemic year, six subgroup A strains and no B strains were isolated. During the next three annual epidemics, subgroup B strains were predominantly isolated, i.e., 8 of 13, 10 of 13, and 13 of 17 strains. However, in the last epidemic year only one strain of 22 isolates belonged to subgroup B, and the remainder belonged to subgroup A. This variability of dominance in the occurrence of different RSV subgroup strains may suggest a possible role of the subgroup-specific immune response in RSV epidemics.