Specific IgM antibodies that persisted for up to four months were detected by radioimmunoassay (RIA) in the sera of 16 (55%) of 29 women with primary infections due to cytomegalovirus (CMV). The RIA for IgM detected primary infections in six (86%) of seven sera obtained within four months of seroconversion. In contrast, IgM antibodies were never detected by RIA in 104 serum samples from 18 women with recurrent infections due to CMV, irrespective of whether intrauterine transmission of virus had occurred. Specific IgM antibodies were also detected in the earliest samples during pregnancy of serum from three (14%) of 21 women whose type of infection with CMV was unknown but who had been delivered of congenitally infected infants. All of these results show that primary infection with CMV in the first trimester of pregnancy can be diagnosed by testing a single serum sample by RIA for IgM antibodies. Attempts to measure IgM antibodies by immunofluorescence gave a high frequency (19 [18%] of 104) of false-positive reactions.