We have studied the effect of three different anti-receptor anti-sera on insulin binding and glucose oxidation by isolated adipocytes. All sera inhibited insulin binding by altering receptor affinity. Titers ranged from 1:5 to 1:5000. Some sera also blocked insulin stimulated glucose oxidation, while others stimulated this function. Both activities were retained purified IgG and F(ab)2 fractions of the antibodies. Thus these antibodies serve as unique probes of both insulin structure and function.