Previous studies have shown that IL-10 may modulate immune responses towards the humoral arm by inhibiting production of cytokines involved in cell-mediated responses. In the present studies, we found that mRNA to IL-10 could be demonstrated in 66% of melanoma cell lines by PCR amplification of reverse-transcribed mRNA and in supernatant of the cell lines by ELISA. Release into the supernatants increased approximately 2-fold each day up to 3 days. The MW of 35S-labelled IL-10 secreted by melanoma cells was similar to that reported in previous studies. In the present studies we also examined whether IL-10 may be responsible for some of the immunosuppressive effects of the melanoma cell supernatants observed in previous studies, by testing whether MAbs against IL-10 could reverse the inhibitory effects of these supernatants. Recombinant IL-10 and melanoma supernatants were found to inhibit production of TNF-α, IFN-γ, IL-2 and mixed lymphocyte reactions but reversal of these effects of melanoma supernatants by MAbs against IL-10 was only seen in the case of TNF-α production. These results extend the range of cell types known to produce IL-10 and indicate that malignancy of certain cell types may lead to unregulated production of IL-10 that could have the potential to modulate immune responses against the tumor.