Rubella virus-infected cells were fractionated by differential and sucrose gradient centrifugations. Rubella virus antigens distributed into all fractions but particulate material in the 100,000 x g pellet was shown to be enriched about two-fold for rubella virus antigen. Similarly, sucrose gradient fractions for rough endoplasmic reticulum and smooth cellular membranes were enriched for rubella virus antigens. The 100,000 x g pellet and the isolated cellular membranes proved to be useful when different fractions were used in solid-phase immunoassays for rubella virus-specific IgG or IgM. These fractions were equal in quality of the semipurified rubella virus preparations in the IgG assays but inferior to those in the IgM assays. However, simultaneous use of 35/25% sucrose fractions from infected and non-infected cells reveals non-specific binding of IgM to the antigens and renders the IgM tests more specific for rubella virus.