A radioimmune precipitation assay (RIPA) for the major internal polypeptide of mammalian type C viruses is described. Gel chromatography and isoelectric focusing resulted in a polypeptide of approximately 30,000 daltons which migrated as a single band in three different dissociating systems as analyzed by polyacrylamide gel electrophoresis. This VP3(gs) protein from murine and feline type C viruses has species specific antigenic reactivities which were not altered by chemical iodination. 125I-labeled VP3(gs) was a sensitive probe for antibodies. By employing limiting concentrations of antibody in RIPA, unlabeled murine or feline VP3(gs) antigen could be quantitated by competition with labeled polypeptide. This double antibody VP3(gs) antigen assay was at least 500 times more sensitive than complement-fixation or immunodiffusion and was specific for type C viruses of the homologous species.