A cohort of 47 renal transplant recipients was studied prospectively for up to one year after transplantation. Cytomegalovirus (CMV) was isolated from 21 of the patients. The first time the virus was isolated seven patients were IgM positive, nine showed a significant rise in IgG titres, and 12 had a four-fold or greater rise in complement fixation titre. There was no significant difference in the time at which virus was first detected following transplantation between patients with primary CMV infection and those with reinfection or recurrent infection. In general, patients with primary infection shed virus consistently over long periods. Those with reinfection or recurrent infection shed virus intermittently or not at all. There were considerable differences between individual patients in the timing and pattern of the immune response. Taken overall, a four-fold rise detected by the complement fixation test correlated best with the onset of CMV shedding in primary infection. There was more variation in the pattern of antibody response in cases of reinfection or recurrent infection, with no single serological test correlating better than the others. It is concluded that serology is of limited value in the detection of active CMV infection after renal transplantation.