Plaque cloning was conducted with MDCK cells on nine influenza B isolates from 1973 to 1977 in Okayama Prefecture, following hemagglutination inhibition test. Hemagglutinin (HA) antigenicity of clones isolated from two out of four B/Gifu/2/73 like isolates in 1973 varied greatly, including B/Kanagawa/3/76 like viruses, and the clones from B/Kanagawa/3/76 like isolate in 1974 also varied greatly, including B/Kanagawa/3/76 like and its variants. One clone from four B/Kanagawa/3/76 like isolates in 1977 indicated the HA antigenicity of B/Gifu/2/73 variant. After B/Gifu/2/73 and B/Kanagawa/3/76 strains were twice cloned by direct plaque to plaque on MDCK cells, these viruses were cultivated in the presence of homologous antibody. Tenth cultivated viruses and their clones indicated the HA antigenicity of each variant. Hemagglutinin antigenicity of both clones from the isolates and the cultivated viruses in the presence of antibody were stable after fifth cultivation in MDCK cells. From these results, it is suggested that the prevarent virus may be pre-isolated by plaque cloning from and cultivation in the presence of antibody of prevarent isolates.