376 sera from Norwegian kidney-transplant recipients and their living, related donors, were tested for the presence of cytomegalovirus antibodies (CMV-ab) by the complement fixation test (CF), and for IgG CMV-ab by the enzyme-linked immuno-sorbent assay (ELISA). Whenever the results of the two methods differed, the sera were also tested by the indirect fluorescence antibody test (FAT). 325 of 376 sera showed corresponding results, giving a concordance of 86 per cent between CF and ELISA, ELISA being the more sensitive of the two, with a sensitivity in the same range as FAT, and high specificity with only 0.3 per cent false positive results. The prevalence of CMV-ab in Norwegian kidney-transplant recipients and their living, related donors, differed very little. The overall CMV-ab seropositivity was 76 per cent. The prevalence increased from 50 per cent at the age of 20 years, to 80 per cent at 30 years and 90 to 100 per cent from 60 years of age.