Peripheral blood polymorphonuclear (PMN) cells from 35 immunocompromised patients (22 heart transplant recipients and 13 AIDS patients) and four normal subjects were tested for the presence of human cytomegalovirus (HCMV) immediate early antigen (IEA) (antigenemia) by indirect immunofluorescence (IFA) and IEA-specific monoclonal antibodies (MAb). PMN samples were tested in parallel for HCMV isolation (viremia) by using MAb to viral early antigens (EA) and the IFA technique 24-48 hr after inoculation onto human fibroblast monolayers. HCMV was isolated from 26 of 83 PMN samples examined: of these, 25 were also positive for HCMV IEA (96% sensitivity). Seven additional PMN samples negative for viral isolation resulted IEA-positive (87.7% specificity). Six of the seven discordant samples were taken from four patients during ganciclovir treatment. The transitory dissociation between positive HCMV antigenemia and negative viremia during antiviral treatment was followed, at the end of the therapy, either by virus clearance and disappearance of IEA-positive PMNs (one patient) or by reappearance of viremia (three patients). Among concordant positive samples, a significant correlation was observed between the number of IEA-positive PMN leukocytes and EA-positive nuclei of infected fibroblasts, when the same number of PMNs were used for both tests.