Human cytomegalovirus (HCMV) can be isolated from peripheral blood leukocytes; however, in vitro, only abortive infection of monocytes, lymphocytes, and granulocytes has been detected. These studies demonstrate that freshly isolated monocytes can be infected with HCMV. Infection of monocytes was not associated with loss of cell viability. The virus replication cycle in monocytes resembled that observed in fibroblasts but the virus yield was approximately 0.1% of that observed in fibroblasts. Transient phenotypical changes occurred in HCMV-infected monocytes. Virus persists in infected monocytes upon differentiation to macrophages, suggesting that monocytes may serve as a carrier of HCMV and a vector for viral dissemination. Differentiated mononuclear phagocytes appear to support a productive HCMV infection. Using a recombinant HCMV strain to express beta-galactosidase, we were able to transduce the bacterial beta-galactosidase gene into monocytes and macrophages.