A sensitive radioreceptor assay was developed for pharmaceutical insulin formulations with the use of rat liver membranes and 125I-insulin. The addition of unlabeled insulin or insulin analogues inhibited the binding of 125I-insulin to the receptors in a concentration-dependent way. Insulin concentrations between 3 x 10(-9) and 3 x 10(-8) M were used for a three-point assay which fulfilled pharmacopoeial statistical requirements for validity of the assay. The coefficient of variation for five assays was 4%. Potency estimates of insulin preparations obtained with the radioreceptor assay correlated well with corresponding estimates from in vivo assays. Both assays also gave similar potency readings for insulin preparations that were partly degraded during storage at elevated temperatures, but the radioreceptor method was more precise.