Three different serotypes of human rotavirus isolates defined in our laboratory were compared by cross neutralization tests with human rotavirus serotypes established in the NIH, U.S.A. The results clearly demonstrated that our three serotypes correspond well to their serotypes Wa, DS-1 and M (or P). Using the three serotype-specific rabbit antisera, all of the 16 strains isolated to date could be assigned to one of those three serotypes. The prevalence of human rotavirus serotypes 1, 2 and 3 among inhabitants of Sapporo and its outskirts was investigated based on the results of neutralizing antibody distribution patterns by age using sera of non-infectious disease patients examined at the Sapporo Medical College Hospital. Neutralizing antibody titers were measured against four strains, KU and K8 (serotype 1), S2 (serotype 2) and YO (serotype 3). The results revealed that serum antibody titers against KU, K8 and YO strains rose with time after birth, reaching the highest antibody distribution levels in either the 3-5-year-old or 6-9-year-old age group, while antibody against S2 strain tended to be lower than that against the other three strains throughout all age ranges examined, with the highest level being shown in the adult group.