Peripheral blood leukocytes (PBL), namely polymorphonuclear leukocytes (PMNL), are the major carrier of human cytomegalovirus (HCMV) in the blood of immunocompromised patients with HCMV viraemia. By using monoclonal antibodies (MAbs) directed against different early and late viral proteins, we showed that the protein accumulating in PBL, originally reported to be an immediate early (IE) gene product, is the 65K lower matrix early structural protein (ep65). This protein is detectable by immunofluorescence before IE proteins during early stages of the replication cycle of HCMV in permissive human embryonic lung fibroblast cells. However, the appearance of ep65 in the nucleus within 1 h post-infection in the presence of cycloheximide indicates that it represents uptake from the virus inoculum rather than newly synthetized protein. The ep65 MAbs staining PBL did not react with Vero cells infected with a recombinant vaccinia virus encoding the major IE gene (IE1) product, whereas MAbs reactive with the 72K major IE protein stained only faintly a small number of infected PBL. A group of four ep65 MAbs was tested in competitive binding assays to show that ep65 possesses at least three distinct epitopes. These were recognized by all four MAbs in AD169-infected Vero cell cultures when fixed with formaldehyde, whereas only one MAb recognizing a distinct epitope was reactive with methanol-acetone (MA)-fixed AD169-infected Vero cells. In formalin-fixed PBL the number of infected cells stained by the four ep65 MAbs was about twofold that found using MA-fixed cells. Using fluorescence-activated cell sorter-purified leukocyte subpopulations from viraemic patients with different levels of viraemia, the ratio of ep65-positive to ep65-negative cells was found to be 1:100 to 1:100,000 for PMNL, and only 1:10,000 to 1:100,000 for mononuclear cells.