Cytomegalovirus (CMV) specific humoral and cellular immunity was investigated in 16 renal allograft recipients with long term graft survival (26-122 months) who were shown to be CMV seropositive before transplantation. Results were compared with healthy individuals with latent CMV infections. Recipients (n = 9) who experienced a symptomatic secondary CMV infection shortly after transplantation (less than 6 months), showed a prolonged but finally temporary suppression of their in vitro lymphocyte memory responses against CMV infected fibroblasts (CMV-FF; median SI: 1.9), a persistence of high antibody titres against intracellular CMV antigens and most of them also had antibodies against CMV membrane antigens (CMV MA). In contrast the recipients (n = 7) who could maintain their CMV in latency after transplantation, had lower antibody titres and their in vitro memory lymphocyte responses against CMV-FF (median SI: 9.3) were comparable to those of the healthy controls (median SI: 11.6). The memory lymphocyte responses against purified CMV virions were depressed in both recipient groups. These results suggest that cellular immunity against CMV infected target cells constitute an important mechanism in maintaining CMV in latency after allografting.