A human islet adenoma was obtained at surgery, homogenized and fractionated by centifuge into nuclear, secretion granule, microsomal and soluble fractions in 0.25 M sucrose. Protein, RNA and immunoreactive insulin were determined in the fractions. By means of polyacrylamide electrophoresis and Sephadex column chromatography, estimates of insulin precursors were made in all fractions. Proinsulin and an intermediate immunoreactive protein were estimated at less than 6% of the total immunoreactive material in the homogenate, nuclear and secretion granule fractions. In the microsome fraction, proinsulin and intermediate protein represented 50-80% of the total insulin-like material while in the soluble fraction, 17-30% of the assayable material was proinsulin + intermediate protein. The total content of insulin was 185 U/g wet weight. The human islet adenoma was compared to islets from other species and the significance of the results discussed.