The results from our investigations concerning the secretory activity of interleukin 10 (IL-10) by two transplantable melanoma cells showed that a spontaneous alteration of the native-melanotic line into an amelanotic form and the tumor progression connected with it, were accompanied by a 6-fold decrease of the IL-10 content in the supernatant of these melanoma line cell culture. Simultaneously, the intracellular content of IL-10 indicated that there was only a small population of IL-10 positive cells, numerically similar in both melanoma cell lines. A comparison of the IL-10 and nitric oxide (NO) secreted by both melanoma lines does not show any correlation between the changes in the content of these substances, which seems to indicate that NO does not act as an autocrine regulator of IL-10 secretion.