IgM and IgG class antibodies to cytomegalovirus (CMV) late antigen were studied in 58 bone marrow transplant (BMT) recipients and their donors using a sensitive enzyme-linked immunosorbent assay (ELISA) and with standard virological and histomorphological techniques. Patients who were CMV-seropositive before BMT had a significantly higher risk for active CMV infection after BMT than seronegative ones (23 of 29 vs. 3 of 26 patients; P less than 1 X 10(-6)). Transplantation of marrow from CMV-seropositive donors was associated with a higher incidence of active CMV infection after BMT than transplantation of marrow from seronegative donors (17 of 28 vs. 9 of 27 patients). Such transplantations also had a significantly higher incidence of grades II-IV acute graft-versus-host disease (23 of 29 vs. 11 of 27 patients; P = 0.007). Following BMT, the evolution of the IgG class CMV antibody response was influenced by the serological status of the marrow donor. First, a fall in IgG class CMV antibody titers during the first month after BMT was seen less often after transplantation of marrow from seropositive donors than after transplantation of marrow from seronegative donors. Second, recipients of marrow from CMV-seropositive donors who developed active CMV infection had an earlier IgG antibody response than those with seronegative marrow donors. These results suggest that the transfer of memory B and T cells occurs with the graft. Failure to mount a sustained IgM or IgG antibody response upon active CMV infection was associated with a fatal outcome.