Abstract: A method for determination of an antigen, which comprises preparation of a onjugate by way of cross-linking of an antibody with an inorganic pyrophosphatase, reacting the resulting conjugate with the antigen to be determined attached to the surface of a solid phase, separating the solid phase from the liquid one, reacting one of said phases with a substrate represented by a salt of pyrophosphoric acid. After carrying out the reaction, a color reagent incorporating a salt of molybdic acid, a surfactant, malachite green dye and a strong inorganic acid is added to the resulting reaction mixture and the content of the antigen is determined from its optical density.