Twenty-seven patients who underwent renal allograft transplants were periodically screened for the presence of IgM and IgG cytomegalovirus (CMV) antibodies. Before transplantation, 17 (63%) had IgG antibodies by complement fixation and 10 (37%) were seronegative. Immunofluorescent IgM CMV-antibodies were absent in all pre-transplant sera. During a follow-up period of 28 to 425 days (median of 115 days), 20 (74%) developed serologic evidence of CMV infection. Of these, 6 (30%) were primary and 14 (70%) secondary (reactivation or reinfection). IF-IgM antibodies were seen in 12 out of 14 patients with secondary CMV infection and in all patients with primary infection. IgM antibodies became negative in one out of 10 IgM-positive patients who were followed-up for 4 months or more.