A method is described for quantitative determination of the insulin binding capacity of human plasma. In this method the insulin binding capacity of non-iodinated insulin is determined. The determination is not influenced by free or antibody bound plasma insulin in the concentrations usually found. It is shown that under the condition used, equilibrium between free and antibody bound insulin is first attained after one week. It is also shown that repeated freezing and thawing of plasma results in a reduction of the insulin binding capacity. Lower detection limit for antibody determination estimated as upper 95 % control limit for pig and ox insulin was found to be 32 and 25 μunits/ml, respectively.