Supernatants from activated human T lymphocytes were highly growth inhibitory for A375 human melanoma cells. Three growth inhibiting polypeptides, transforming growth factor beta 1 (TGF-beta 1), interferon-gamma (IFN-gamma), and oncostatin M, were isolated from the acid-soluble fraction of serum-free T cell-conditioned medium and purified by gel permeation chromatography and reverse-phase high performance liquid chromatography in volatile solvents at acid pH. The purification was monitored in a growth inhibition assay. The release of TGF-beta 1 biologic activity by and the purification of IFN-gamma from the medium of activated human peripheral blood T lymphocytes have been reported. We now describe the isolation of oncostatin M from the conditioned medium of activated human T cells. The concentration of oncostatin M required for half-maximal inhibition of A375 melanoma cells was approximately 4 pM when assayed in the presence of 10% fetal bovine serum. The purified oncostatin M had an apparent m.w. 28,000 and an amino-terminal sequence that was identical with the sequence of oncostatin M isolated from supernatants of macrophage-like cells. Suboptimal concentrations of TGF-beta 1 in combination with suboptimal concentrations of IFN-gamma or oncostatin M resulted in synergistic antiproliferative responses for A375 cells (1.9 and 3.1 times the expected additive responses, respectively). Combinations of oncostatin M and IFN-gamma added simultaneously to A375 cells caused an additive growth inhibitory response. These results demonstrate that oncostatin M is a novel lymphokine, and its interaction with other cytostatic polypeptide growth inhibitors may play a role in the immune regulation of tumor cell growth.