Recently, severe epidemics of diarrhea among both adults and children in China have been associated with an agent called adult diarrhea rotavirus (ADRV). We have studied ADRV from two areas of China (Jinzhou and Guangxi) and compared them with prototype group A, B, and C rotaviruses. The viral genomes were compared by electrophoresis of the RNA genome segments in polyacrylamide gels, and the antigenic relatedness of these viruses was examined by immune electron microscopic studies using virus preparations of either double- or single-capsid particles. Hyperimmune antisera (guinea pig and rabbit) to one strain of ADRV reacted with high titers with other strains of ADRV and with single-shelled capsids of a porcine group B rotavirus. The antisera to ADRV did not react with either group A or group C rotaviruses. Antisera to either bovine or porcine group B rotaviruses reacted with ADRV but not with the group A or group C viruses. Antisera to group A or group C virus reacted only with their respective homologous virus. These results and those of immunofluorescence studies place the human ADRV rotaviruses from China among the group B rotaviruses. We also report the development of an ELISA to detect ADRV; this ELISA should be useful for defining the epidemiology of these recently described rotaviruses.