There is accumulating evidence for interaction of Epstein-Barr virus (EBV) and human immunodeficiency virus (HIV): EBV and HIV may coinfect B-lymphocytes; the prevalence of active EBV is increased in active homosexuals; and EBV-related B-cell lymphomas occur frequently in AIDS patients. We have shown that cord-blood B-lymphocytes may become infected by HIV once preinfected and transformed by EBV. The present paper, in addition, summarizes mechanisms of transactivation of an HIV LTR construct by an EBV gene product. Also, preliminary data are presented on the activation of an EBV promoter by the human T-cell leukemia virus (HTLV-1).