Interactions between human immunodeficiency virus type 1 (HIV-1), the etiologic agent of AIDS, and human cytomegalovirus (CMV), a frequent opportunistic agent in AIDS, were studied in vitro. Coinfection of H9 cells with HIV-1 enhances productive CMV infection, as measured by immunofluorescence using monoclonal antibodies to late CMV proteins, slot-blot hybridization for CMV DNA, and cytopathic effects of CMV on human embryonic lung cells. Experiments using vaccinia virus recombinants and Jurkat cells transfected with the transactivating (tat) gene of HIV-1 suggest that this enhancement is not mediated primarily by the tat protein. In addition, coinfection of H9 cells or a monocyte cell line with CMV and HIV-1 results in enhanced HIV-1 replication, as measured in a virus-yield assay or by radioimmunoassay for the p24 antigen of HIV-1. The interactions between HIV-1 and CMV are thus bidirectional.