IDS is a soluble glycoprotein product of activated T cells that inhibits lymphocyte proliferation induced by antigens and by lectin mitogens. This immunosuppressive lymphokine has been distinguished from lymphotoxin, Proliferation Inhibitory Factor, Colony Inhibitory Factor, Macrophage Inhibitory Factor, and interferon. Using IDS partially purified by isoelectric focusing from culture supernatants of concanavalin A-stimulated human peripheral blood mononuclear cells (PBMC), we investigated IDS inhibition of T cell proliferation with respect to the interleukin pathway. At concentrations that produced 75-90% suppression of proliferation in PHA-stimulated PBMC cultures, IDS caused no decrease in interleukin 2 (IL2) production (determined by bioassay) or in IL2 receptor expression (determined with anti-Tac antibody). Moreover, adding exogenous IL2 to IDS-inhibited cultures failed to restore proliferation. IDS inhibited growth of several IL2-dependent and IL2-independent cell lines, and suppressed proliferation of PBMC induced by the phorbol ester TPA (12-0-tetradecanoyl-13-phorbol acetate) or by the calcium ionophore A23187, thus distinguishing its mechanism of action from that of cyclosporin A, dexamethasone, OKT11A antibody, and other inhibitors. These data extend earlier findings that IDS acts late in G1 phase of the cell cycle, and provide evidence that IDS inhibits T cell proliferation through an IL2-independent mechanism.