Serotyping of human rotaviruses (HRVs) by neutralization requires the previous adaptation of strains to growth in cell cultures, which is often unsuccessful. By using the solid-phase immune electron microscopy (SPIEM) technique with protein A and type-specific, cross-adsorbed, polyclonal immune sera, we divided 40 previously typed culture-adapted strains into the same four serotypes distinguished by neutralization, but HRV strains could also be typed directly on stool extracts. Of 171 HRV strains tested by SPIEM, 163 were typed as a single serotype, two were shown to be mixed serotypes, three could not be typed since they were partially antibody-coated, and three were lacking the common group antigen (atypical rotaviruses or pararotaviruses). In addition, strains of serotype 4 could be classified by SPIEM into two subtypes, 4A and 4B. Overall, strains of serotype 1 were detected in nearly 50% of the 171 HRV-positive stools examined by SPIEM, strains of either serotype 2 or 4 in about 20%, and strains of serotype 3 in nearly 10%. Although the data were too scarce to allow for definite conclusions, the prevalence of the four serotypes appeared to change with the geographic area and the year.