Abstract: Quantitative and qualitative measurement of antigenic materials present in blood serum or other biological fluids or cells is effected by an extremely precise and sensitive colorimetric immunoassay process. This process comprises adding to the sample to be tested a colorimetric immunoassay agent which comprises an adduct of (1) an antibody for the antigen to be tested and (2) a chromoprotein which is capable of chemically coupling to the antibody by covalent bonding and which chromoprotein imparts to the adduct a characteristic absorbance spectrum in the visible light wavelength range. After reacting this colorimetric immunoassay agent with the antigen to form a reagent-antigen complex, either in the complex-containing fraction or a fraction containing unreacted reagent can be analyzed using standard colorimetric equipment and procedures to provide a fast, simple and direct measurement of the antigen concentration in the sample.