The Homogeneous Coenzyme-immunoassay for quantitation of insulin in a solution has been developed. The method relies on the comptitive binding of free and coenzyme (NAD)-labeled insulin by insulin antibodies (guinea pig anti-porcine insulin). NAD-insulin conjugates were obtained by a covalent binding of NAD-N6-[N-(6-aminohexy1)acetamide] and insulin in the presence of water-soluble carbodiimide. The NAD-insulin conjugaties were cycled in the coenzyme regeneration system Which included horese liver alcoholdehydrogenase (AIH) and its coupled substrates-cyclohexanol and P-nitroson-N, N-dimethlaniline (NIMA)- and the cycling rate was used to measure the conjugate concentrations. the cycling was inhibited when the lignad (insulin molecule) was bound to the insulin antibodey. The unbound insulin-NAD conjugate could be measure in the presence of the protein-bound cojugate. The Sensitivity of the proposed method of insulin determination in limited by the value of the antigen-sntibody binding constant and permits and accurate measurement of concentrations up to about 10-7M.