Human and viral interleukin-10 (IL-10) possess growth factor activity for human B cells and may act as autocrine growth factors in B cell malignancies. To study this possibility we have measured viral (v) and human (h) IL-10 expression in EBV-positive and negative B lineage tumors and tumor cell lines. Previous studies demonstrated IL-10 expression in cell lines and now we describe the pattern of IL-10 expression in primary Burkitt's lymphoma (BL) tumor biopsies and in BL lines of defined phenotypes. vIL-10 was expressed during the lytic (productive) phase of EBV infection, but not during virus latency. Although hIL-10 was expressed in the majority of B cell lines, it was not expressed in two BL biopsy specimens. Expression of hIL-10 did not correlate with the presence of EBV, but was associated with the differentiation state of the B cell line. Thus, vIL-10 may enhance the persistence of B cells infected at sites of virus replication, and while hIL-10 may be a factor in the growth both in vivo and in vitro of some BLs and EBV-transformed B cells, it is not an absolute requirement.