A rapid assay is described for detection of cytomegalovirus in peripheral blood lymphocytes. It consists of an indirect immunofluorescence technique for detection of cytomegalovirus antigens by means of a monoclonal antibody directed against early viral coded proteins. This assay was compared with the conventional cell culture system. Patients transplanted between December 1986 and May 1988 were studied, and of 27 patients identified, two were excluded due to early graft failure. A total of 320 blood specimens obtained were studied, and from 12 patients cytomegalovirus was isolated in at least one specimen by conventional cell culture (in total 25 specimens). Results were available with the new technique within 6 h, whereas the cell culture took an average of 13 days to develop the typical cytopathic effects changes. Sensitivity and specificity compared with that of viral isolation in conventional cell culture was 92% and 95%, respectively. This technique provides an accurate and rapid diagnosis of cytomegalovirus infections, and allows specific antiviral therapy to be started earlier.