Plasma membrane vesicles from rat adipocytes were treated with dimethylmaleic anhydride to remove extrinsic proteins and then used to immunize rabbits. Immunodiffusion experiments performed in agarose containing Triton X-100 (0.1%) revealed a precipitin reaction between anti-membrane serum and the detergent-solubilized proteins from either intact adipocyte plasma membranes or membranes extracted with dimethylmaleic anhydride. Anti-membrane serum caused cytolysis of intact adipocytes and this effect could be eliminated by incubating the serum at 56 degrees C for 30 min to inactivate complement. Heat-inactivated anti-membrane serum caused a significant increase in 14CO2 production from D-[1-14C]glucose in intact fat cells and the partially purified immunoglobulin fraction from anti-membrane serum markedly stimulated 3-O-methylglucose transport. Maximum activation of transport occurred at a 1:5 dilution and was not additive to that achieved by a maximal dose of insulin. Under these conditions, the anti-membrane immunoglobulin fraction had no effect on the binding of 125-i-insulin to fat cells at all concentrations of the hormone tested. The data are consistent with the hypothesis that activation of hexose transport in these studies results from the binding of immunoglobulin directly to the transport system, although an indirect action is also possible.