Detection of cytomegalovirus (CMV) antigenemia was compared with shell vial centrifugation cultures for rapid detection of CMV infection. In a prospective study, 59 CMV seropositive patients were monitored weekly during the first 100 days after allogeneic marrow transplantation for virus excretion from urine, throat, and blood and for antigenemia by direct staining of peripheral leukocytes using an antibody pool directed against pp 65. Antigenemia was present in 21 of 22 patients with culture-proven CMV infection and in 3 of 37 without culture-proven CMV infection (sensitivity 95%, specificity 91%). The median time of onset of antigenemia and shell vial cultures was day 47 and 55 after transplant, respectively (P = .0006). Among patients who developed CMV disease without preceding cultures, antigenemia was detected in all patients with CMV pneumonia (N = 6) and in two of three patients with gastrointestinal disease by a median of 10 and 7 days, respectively, before the onset of disease (P = .0002). Levels of antigenemia were significantly higher in patients with disease or viremia than in patients with excretion from urine or throat (P less than .05). Whether the antigenemia assay is more sensitive than rapid culture methods to focus antiviral prophylaxis in marrow transplant patients must be determined in controlled studies.