We investigated the effect of the specific sulfhydryl blockers, N-ethylmaleimide (NENI) and dithiobisnitrobenzoic acid (DTNB), on 125I-insulin binding to adipocytes, adipocyte plasma membrane and IM-9 lymphocytes. NEM markedly inhibited the insulin binding to intact cells to lower than50% of thc control whether NEM was added bcfbre or simultaneously with the 125I-insulin to the cell suspension, but DTNB, showed no such effect. The inhibition was temperature, time and concentration dependent. Scatchard analysis of the binding indicated that NEM affected the affinity rather than the capacity. However, both reagents did not affect the insulin binding to the adipocyte plasmamcmbrane. We considerd that NEM did not act on the binding mechanism of the plasma membrane receptor but intracellularly, since NEM can freely cross the adipocyte plasma membranc but DTNB cannot.