In previous studies, cultured melanoma cells were shown to have a suppressive influence on the induction of cytotoxic T cells. Our investigation of the mechanism of these effects revealed that supernatants from certain cultures of melanoma cells contained inhibitory activity against the production of interleukin 2 (IL 2) from phytohemagglutinin (PHA)-stimulated cultures of lymphocytes. These supernatants did not inhibit interleukin 1 production, and also did not inhibit the mitogenic activity of performed IL 2 on IL 2-dependent target cells. Production of the inhibitory activity could be reduced by inhibitors of protein synthesis, but this activity was not inhibited by digestion with the proteolytic enzymes trypsin or pronase. Gel filtration analysis of tumor supernatants revealed that the majority of the inhibitory activity was detected in fractions of approximately 44 and 7 Kd. The addition of supernatants with inhibitory activity to PHA-stimulated cultures of lymphocytes was associated with reduced transition of cells from G1 to S phase of cell division, which could be reversed by the addition of IL 2. Preliminary studies suggest that the release of the factor(s) from melanoma cells may be related to rapid progression of tumor growth in patients, and therefore may be of prognostic significance in tumor host relationships.