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Abstract
A kinetic model is put forward for the study of the antigen‐antibody reactions involved in the coated tube radioimmunoassay (RIA) of androstendione. Twenty experiments were conducted to determine the influence of initial concentrations, ionic strength, viscosity, and temperature on the substitution reaction of 125I‐androstendione (M) by unlabelled androstendione (Q) in the immunocomplex PM (P = anti‐androstendione antibody). The results obtained are in line with the proposed model. The concentration of radioactive immunocomplex is directly proportional to the initial concentration of labelled androstendione and independent of the concentration of unlabelled androstendione, ionic strength, and viscosity. The reaction is not diffusion‐controlled.