Infection with human cytomegalovirus (HCMV) after allogeneic bone marrow transplantation (BMT) was studied in 12 HCMV seronegative recipients of marrow from seropositive donors by weekly monitoring of cultures, expression of HCMV antigenemia (pp65) in granulocytes, polymerase chain reaction (PCR) on HCMV-DNA in granulocytes and IgM and IgG anti-HCMV antibodies. Eight patients remained negative in all tests as did 33 HCMV seronegative recipients of marrow from seronegative donors. In four patients, a transient expression of HCMV antigen pp65 in granulocytes from peripheral blood, together with a positive PCR on HCMV-DNA from the same samples were found without positive cultures, seroconversion or expression of other HCMV antigens in granulocytes. The data indicate the presence of an abortive HCMV infection in these four patients.