Fresh plasma and peripheral blood mononuclear leukocytes were obtained from 11 patients with acquired immunodeficiency syndrome (AIDS). All of these patients were seropositive for antibodies to HTLV-III/LAV and Epstein-Barr virus (EBV). Plasma and uncultured peripheral blood leukocytes from these patients were tested for the presence of infectious EBV and membrane antigen (EBV-MA). Transforming EBV was isolated from the cell-free plasma of 4 patients with AIDS exhibiting EBV-MA in uncultured cells. No virus was detected in plasma from the healthy donors or from the remaining AIDS patients. No neutralizing antibody to EBV was detected in the 4 patients whose plasma contained EBV. In contrast, neutralizing antibody was present in the plasma of EBV-seropositive healthy donors and in 7 other patients with AIDS. One of the AIDS patients whose plasma contained EBV recently developed Burkitt's-type lymphoma. The production of transforming EBV could be induced either directly or indirectly by HTLV-III/LAV infection, and also could be involved in the frequently observed B cell malignancy associated with AIDS.