Prolonged incubation of IM-9 lymphoblastoid cells with concanavalin-A (Con-A) reduces insulin binding at concentrations of the lectin that neither agglutinate cells nor acutely inhibit insulin binding. Scatchard plot analysis suggested that Con-A treatment reduces insulin receptor number. Con-A-induced reduction in insulin binding resembles insulin "down regulation" in the time required for appearance and reversal of the binding alteration. The effect of Con-A on insulin binding was inhibited when the sugar alpha-methyl mannoside was incorporated into the incubation medium at the same time as Con-A. Even when the sugar was added after hours of Con-A exposure, the depressed insulin binding was reversed proportional to the reduction of Con-A binding to the cells by the sugar. This latter observation suggests steric hindrance as the mechanism for Con-A-induced reduction in insulin binding. The slow association rate of Con-A to the cells under the conditions of the experiments probably explains the prolonged exposure time to Con-A required for inhibition of insulin binding. Additional support for steric hindrance was observed in experiments in which insulin was "trapped" on cells after exposure to Con-A. Bivalent succinyl Con-A was only 10% as effective as the natural tetravalent lectin in reducing insulin binding. Cytoskeleton and protein synthesis inhibitors failed to alter the Con-A-induced reduction in insulin binding.