Interleukin 10 (IL-10) is a novel lymphokine which exhibits strong DNA and amino acid sequence homology to BCRF1, an open reading frame in the Epstein-Barr virus genome. Using a wide panel of EBV positive and EBV negative cell lines, it has been shown that EBV positive B cell lines derived from patients with AIDS and Burkitt's lymphoma (AABCL) secrete large quantities of B cell IL-10, compared with EBV-positive B cell lines obtained from patients with undifferentiated lymphomas of Burkitt's and non-Burkitt's types. In contrast, EBV-negative B cell lines do not express IL-10 by Northern blot analysis, ELISA or even PCR. B cell IL-10 is confined to a narrow window in the B cell differentiation pathway, and whereas IL-10 expression is detected in mature and preplasmacytic stages, none of the pro-B, pre-B, or myeloma cell lines produce IL-10. EBV exerts direct effect on the production of B cell IL-10, and purified tonsillar B cells infected with EBV were triggered to secrete IL-10. The large amount of IL-10 secreted by B cells derived from AIDS-related lymphomas suggests that HIV-1 also exerts direct effect on IL-10 secretion. B cell IL-10 may function as autocrine growth factor for B cell lymphomas, and both IL-10 and BCRF1 seem to be involved in the pathophysiology of non-Hodgkin's lymphomas.