An indirect enzyme immunoassay (EIA) for the determination of IgG and IgM antibodies to mumps virus is described. Viral antigens and control antigens were adsorbed onto polystyrene microtiter plates, and antibodies attached to the antigens were detected by subsequent binding of commercial peroxidase-labeled antibodies to the heavy chains of human IgG and IgM immunoglobulins. A comparison of antibody titers obtained by the EIA and by indirect immunofluorescence test showed a close concordance between these two tests, with EIA, however, being more sensitive. Occasional cross-reactions between mumps and parainfluenza antibodies were detected in the IgG antibody test but not in the IgM antibody test. In sera from 84 patients with mumps infection, all cases were diagnosed by the EIA IgM antibody assay, 96% from the first serum specimen. Mumps was diagnosed by complement fixation (CF) in 71% of these cases: unclear or erroneous results with parainfluenza titer increases in 10% and no diagnosis in 18% of the cases. The EIA IgM antibody assay was thus better than the CF test for the diagnosis of acute mumps infection.