A solid phase radioimmunoassay for the detection of mumpsspecific IgM antibodies (MACRIA) is described. In 22 cases of clinically suspected mumps infections confirmed by conventional serological methods, IgM antibodies to mumps virus were detectable. On the other hand, no mumpsspecific IgM antibodies were present in 9 cases of suspected mumps that could not be confirmed by classical serology (seroconversion or significant titer rise in CFT or NT). Neither could IgM antibodies to mumps virus be detected in the sera of 19 rheumatoid factor positive patients nor in those of 20 healthy laboratory personnel. 20 patients from a rubella outbreak who had been found to be positive for antirubella IgM in a comparable MACRIA system showed no IgM response in mumps-MACRIA. In one case of mumps infection during pregnancy where mumps virus was isolated from the salivary glands by throat swab, IgM antibody response was delayed and markedly weaker than in all other patients with a positive IgM response. For this patient, who showed a significant titer rise in NT and seroconversion in CFT to mumps virus, the possibility of reinfection is discussed.