An immunosorbent technique was developed to attenuate cross-reactivity of a polyclonal antiserum against a 4(2) (rho-carboxyphenylazo)-1,3,5[10]-estratrien-3,16 alpha,17 beta-triol-bovine serum albumin conjugate. The chromatographic separation of antiserum through stationary phases having either rho(carboxymethyl)phenylazo-phenol or rho(carboxymethyl)-phenylazo-2-naphthol side residues reduced the antiserum avidity, while increasing the apparent antiserum affinity and decreasing the residual cross-reactivities against heterologous ligands. The highly specific antiserum obtained allowed the development of a competitive binding assay over an extended analytical range, which opens up the possibility of direct measurement of estriol from the early pregnancy to delivery. The significance of the attenuation of antiserum cross-reactions after affinity chromatography is discussed with reference to epitope-paratope interaction in the case of small endogenous molecules like estrogens.