Reactivation of human cytomegalovirus (HCMV) from latency occurs in immunosuppressed individuals and infection is itself immunosuppressive. To better understand the basis for this virally induced impairment of immune function, we have analyzed virus-leukocyte interactions by in situ hybridization. We detected viral DNA in 12 viremic patients in the mononuclear cell population, predominantly in cells identified as monocytes by their morphology and by labelling the cells with a monocyte specific monoclonal antibody prior to in situ hybridization. We detected immediate early RNA in infected cells at frequencies comparable to DNA (10(-3) to 10(-5)). By contrast, no viral transcripts were detected in polymorphonuclear cells and viral DNA was inclusively cytoplasmic in accord with the interpretation that this cell type harbors HCMV in phagosomes. These findings in vivo continue to suggest that infection of monocytes plays an important part in the immunosuppressive effects of HCMV infections.