A streptococcus, AW 43 strain, was found to bind nonspecific serum inhibitors of rubella virus hemagglutination (HA). This was demonstrated by titration of nonspecific HA inhibitors and by immunoelectrophoresis. Absorption of sera with the mixture of AW 43 cells, which bind IgA in addition to nonspecific HA inhibitors, and AR 1 cells, another strain of streptococci which bind IgG, removed nonspecific HA inhibitors, IgG, and IgA simultaneously, leaving behind IgM and a trace of IgA. Pretreatment of sera with those streptococcal cells prior to the rubella hemagglutination inhibition (HI) test enabled to circumvent kaolin treatment of sera, which partially removes IgM antibodies, and to determine exclusively the early-appearing antibodies. The rise and fall of the HI antibodies thus determined correlated well with that of the IgM antibodies determined by enzyme-linked immunosorbent assay (ELISA). Thus, this modified rubella HI test may be useful for serodiagnosis of recent rubella virus infection.