Posttransplant patients undergoing prolonged cyclosporine A (CsA) immunosuppressive therapy have been reported to have increased incidence of Epstein-Barr virus (EBV)-associated lymphoproliferative disorders. We undertook experiments to analyze the possible actions of CsA during EBV-infection of human peripheral blood mononuclear cells (PBMC). EBV-infected B cells cultured with CsA demonstrated increased EBV B-cell outgrowth as compared with those cultured without CsA. PBMC, after infection with EBV and CsA treatment, demonstrated increased interleukin-6 (IL-6) activity in the culture supernatant. The induction of IL-6 appears to differ within the various lymphocyte populations. In monocytes, IL-6 expression appears preferentially induced by EBV and is initiated by the binding of the two major virion glycoproteins, gp350 and gp220. Expression of IL-6 in T cells appears to be due mainly to CsA. B cells also express IL-6 after EBV exposure, but not after CsA treatment. EBV-immortalized B-cell lines cultured with CsA exhibited both an increased number of cells expressing viral lytic-cycle antigens and increased amounts of lytic-cycle proteins. IL-6, which is induced by CsA in PBMC, was also capable of inducing the lytic viral cycle in several EBV-immortalized cells. CsA, in promoting both increased numbers of lytic EBV B cells and an EBV paracrine factor, IL-6, within the microenvironment of EBV B cell:T cell and EBV B cell:monocyte interactions, may result in increased EBV B-cell immortalization and ultimately lead to the promotion of B-cell lymphomas in immunosuppressed patients.