Summary Radio-immunoassay of insulin gives the same results whether immunoprecipitation or chromatography is used. The effects on the assay systems of interference with equilibrium, and the influence of human anti-insulin sera on immunoprecipitation are described. It is suggested that present radio-immunoassays should give similar plasma insulin levels in man if similar reference standards insulins are used. The immunological potency of several human insulins in general use is compared.