Abstract: The present invention relates to a simple and accurate method to be applied in immunoassay technique for separating the free ligand from the antibody-bound ligand.The method involves the use of a solvent extraction operation which at equilibrium provides the formation of two distinct phases the free- and bound-fraction. The solvent to be utilized should be slightly water miscible or completely water immiscible, having the property of marked selective extraction power toward the gamma-labelled constituent to be determined. The method is applicable in a liquid-liquid system, a solid-liquid system and solid-solid system.The method is useful for radioimmunoassay, free radical assay or metallo-immunoassay, being most versatile having also the advantage that as a result of the free ligand being extracted into the solvent, the determination of the quantity of gamma-labelled substance in the free and/or bound fraction compares very favorably with the known prior art methods in immunoassays.