Charcoal premixed with dextran of average molecular weight 80,000 almost instantly adsorbs free insulin but rejects antibody-bound insulin. The use of such dextran-coated charcoal makes simpler and more rapid the immunoassay of insulin in biologic fluids, using radioisotope dilution with 131I-insulin and “biopsy” of the insulin pool by antibody to insulin. The procedure here described yields a straight line graph when insulin added is plotted against insulin recovered.