The progression of tumors such as renal cell carcinoma (RCC), despite the presence of substantial lymphocytic infiltrates (TIL), suggests that the ability of the local immune response to control tumor growth is impaired. Cytokine gene expression was examined to further investigate the nature of this response. Initial studies were performed with frozen tumors using PCR-assisted mRNA amplification with cytokine-specific primers for interleukin 10 (IL-10), interleukin 2 (IL-2) and interferon-gamma (IFN-gamma). IL-2 mRNA was not detected, despite the presence of T cells as defined by the expression of CD3 gamma mRNA. In contrast, mRNA for IFN-gamma was expressed in 4/9 and for IL-10 in 5/9 tumors. To confirm this, 5 fresh tumor specimens were examined, and PCR demonstrated that IL-10 mRNA was detectable in 4/5 tumors from which RNA was isolated at the time of nephrectomy. In these experiments multiple cycles and dilutions were employed to semi-quantitate the expression of IL-10. To identify potential sources of this cytokine in the tumor bed, IL-10 mRNA expression in freshly isolated lymphocytes and tumor cells, TIL lines, cultured RCC and established RCC lines was examined. Our studies demonstrate that within the tumor TIL may be one source of IL-10. Lymphocyte-enriched populations from 4/5 tumors expressed IL-10 mRNA as did 4/6 freshly isolated tumor cell preparations. IL-10 gene expression was not detected, however, in tumor cells after one passage in vitro in short-term cultured RCC tumor cells (passages 2-5) or in established RCC tumor cell lines. Finally, 4/9 CD4+ and 2/5 CD8+ TIL lines expressed IL-10 mRNA either constitutively or after stimulation with anti-CD3 antibody. This finding was associated with IL-10 production in vitro. Our studies demonstrate that IL-10 mRNA is frequently present in RCC tumors and may originate from the tumor-infiltrating mononuclear cell population.