125I-Concanavalin A was found to bind to purified Golgi membranes in a manner which was time-dependent, proportional to the amount of Golgi protein, saturable, and inhibited by alpha-methyl-D-glucoside and unlabeled concanavalin A. Approximately 2 nmol of 125I-concanavalin A were bound per mg of membrane protein with a mean Ka of 0.2 x 10(7) M-1. After binding of concanavalin A to the membranes, all unbound lectin was removed by repeated washing prior to assay of galactosyltransferase activity to assure that bulk concanavalin A would not bind to the sugar acceptor during the enzyme assay. Therefore, the effects of the lectin were not due to altered acceptor concentrations in the assay. As a result of concanavalin A binding the activity of the galactosyltransferase of these membranes was enhanced by 70 to 100%. The enhancement was abolished by treatment of the concanavalin A-bound Golgi with alpha-methylglucoside. Kinetically this stimulation was reflected as an altered Km for N-acetylglucosamine and UDP-galactose. The data are suggestive that the stimulation of the enzyme results indirectly from perturbation of the membranes on binding of the lectin.