The presence of cell surface rubella antigen was used to verify and monitor viral replication in Vero cell monolayers. Viral antigen was observed in infected cells by the adherence of Staphylococcus aureus sensitized with immune anti-rubella sera. The staphylococci specifically bound to infected cells were Gram-stained and observed using light microscopy. The minimum titer of IgG antiviral hemagglutinin required for sensitizing the bacteria was 3.9 IU/ml. The specificity of the assay was demonstrated by treating the infected cells with bacteria sensitized with normal sera, by treating the mock-infected cells with staphylococci sensitized with either immune or normal sera, and by sensitizing the bacteria with immune sera from which anti-rubella antibodies had been removed. Viral antigens were detected from day 2-9 post-infection. The sensitivity of the assay in verifying and monitoring viral propagation was comparable to the titration of viral particles of hemagglutination. The assay is specific, rapid, simple and can be performed in laboratories with minimal equipment.