The molecular and antigenic properties and synthesis of the structural proteins as well as the virus-specific RNAs of rubella virus were analyzed. Virions contain three major polypeptides--E1 (relative molecular weight [Mr] 58,000), E2 (Mr 42,000-47,000), and C (Mr 33,000). E1 and E2 are glycosylated and located externally on the viral membrane. C is associated with the genomic RNA to form the nucleocapsid. E2 occurs in two forms, E2a and E2b; the protein moieties of the two are indistinguishable. E1 is the viral hemagglutinin. IgG antibodies react with all the structural proteins, whereas IgM and IgA antibodies react predominantly with E1 and C proteins, respectively. After rubella vaccination, the reactivity of IgG antibodies matured slowly and reactivity of IgM and IgA antibodies remained at a low level. Rubella virus contains a 40S (Mr approximately 3.8 X 10(6)) single-stranded RNA with a 5' cap structure and a 3' poly(A) tract. In infected cells a 24S (Mr approximately 1.2 X 10(6)) subgenomic, polyadenylated mRNA is synthesized; it codes for a precursor (Mr 110,000 [p110]) to the structural proteins. The gene order in the 24S RNA is NH2-C-E2-E1-COOH. The overall structure and strategy of gene expression of rubella virus appears to be similar to that of the alphaviruses of the Togaviridae family.