Gangliosides are known to be actively shed from tumour cell membranes, and increased levels of circulating gangliosides may cause tumour-induced T-lymphocyte immunosuppression in vivo by interfering with the actions of interleukin-2 (IL-2). We have investigated the effect of gangliosides on the interaction of IL-2 with its cell surface receptors (IL-2R). Gangliosides inhibited IL-2-stimulated proliferation in synchronized populations of the IL-2-dependent cell lines CTLL-2 and HT-2. The immunosuppressive effect was most effective when gangliosides were added during the first 4 hr after IL-2-stimulation, indicating that they acted early in the IL-2 signalling pathway. Inhibition could be completely overcome by exogenous IL-2, suggesting that gangliosides inhibited growth solely by competing with IL-2R for available IL-2. In support of this proposal, gangliosides induced a concomitant dose-dependent decrease in binding of [125I]IL-2 to high-, medium- and low-affinity IL-2R. Ganglioside-treated cells recovered their high-affinity [125I]IL-2 binding after washing. The glycolipids also prevented chemical cross-linking of [125I]IL-2 to the p55/p75 complex, as well as to both IL-2R alpha (p55) and IL-2R beta (p75) independently. A thin-layer chromatography overlay technique was used to demonstrate that IL-2 binds directly to gangliosides, but not to simple neutral glycolipids or acidic lipids. Taken together, these findings indicate that gangliosides directly block the interaction of IL-2 with IL-2R, and may explain, in part, the immunosuppressive activities of gangliosides in vivo.