Specific IgM class antibody production was studied in different groups of patients with characterized cytomegalovirus (CMV) infections using a radioimmunoassay (RIA). In pregnant women, IgM antibodies were detected only following primary infection and generally persisted less than 4 months. The demonstration of CMV-specific IgM during pregnancy is therefore diagnostic of recent primary CMV infection. In patients with symptomatic CMV infections, the appearance of IgM antibody was shown to be closely related to the onset of symptoms and coincided with production of complement fixing (CF) antibody. IgM antibodies were at maximum levels 3-4 weeks after presentation but generally declined to low or undetectable levels by 3-4 months. The significance of the results of testing for CMV-specific IgM in relation to clinical and other serological findings in these patients is discussed. IgM antibody production was also demonstrated in renal transplant patients with primary infections and in 6 of 21 recipients with secondary infections. In both groups the antibodies became detectable 3-6 weeks after transplantation but the titres were much higher following primary infection. IgM antibodies persisted throughout follow-up periods of up to 2 years after transplantation in some cases.