On the basis of in vitro experiments, it has been suggested that cells of hematopoietic origin play a major role in the pathogenesis and latency of human cytomegalovirus (HCMV). To elucidate the in vivo importance of hematopoietic cells in acute HCMV infection, tissue sections from various infected organs were investigated by immunohistochemical double-labeling analyses. Monoclonal antibodies directed against distinct viral and cellular antigens were used to identify infected macrophages, polymorphonuclear cells, and lymphocytes. Macrophages and polymorphonuclear cells were targets for HCMV infection in different tissues. Viral proteins representing all stages of permissive HCMV infection were detected in macrophages, suggesting that these cells support the complete viral replication cycle. In polymorphonuclear cells, viral gene expression was restricted to the immediate early phase, indicating that these cells are abortively infected. These findings suggest that macrophages play an important role in the hematogenous spread of HCMV into solid organs.