Human interleukin-10 (h-IL-10) is a pleiotropic cytokine with stimulatory activity on B-lymphocytes. Recent evidence indicates that infection with Epstein-Barr virus (EBV) induces h-IL-10 production in B-cells and that this cytokine may contribute to EBV-induced B-cell transformation. It is not known whether h-IL-10 induction by EBV correlates with distinct phenotypic features of the infected cells or with the expression of particular viral genes. We have approached these questions by investigating the expression of h-IL-10 mRNA in a panel of B-cell lines including: in vitro EBV-transformed lymphoblastoid cell lines (LCLs), EBV-carrying Burkitt lymphoma (BL) lines, EBV-negative BL lines and their sublines infected with different EBV strains, or transfected with the transformation-associated viral gene. h-IL-10 mRNA was detected by reverse-transcriptase-assisted (RT)-PCR in a subset of EBV-negative BLs and in all EBV-positive BL lines and LCLs investigated except Daudi. This cell line carries an EBV nuclear antigen (EBNA)-2 gene-defective virus strain. h-IL-10 mRNA was induced by conversion of 3 EBV-negative and h-IL-10-negative BL lines (BL41, BL47 and BL49) with the transforming, B95.8-derived EBV strain. P3HR-I virus convertants that do not express the viral EBNA-2 and the EBV latent membrane protein (LMP)-1, and fail to progress towards a LCL-like cell phenotype, showed no evidence of h-IL-10 up-regulation. Expression of LMP1 was sufficient to induce h-IL-10 mRNA in transfected sublines of the EBV-negative DG75 and BL41 cell lines, whereas expression of EBNA1, 2, 5, or 6 had no effect. h-IL-10 was detected in the culture supernatants of the LMP1 transfectants by specific ELISA assays. The present findings confirm the role of LMP1 in the transactivation of a wide variety of cellular genes which may be involved in EBV-induced B-cell transformation.