An indirect immunoprecipitation experiment was developed to characterize the antigenic determinants of the hCG-binding protein and the rat testicular LH/CG receptor. The hCG-binding protein preincubated with 125I iodo hCG was immunologically precipitated by the antibodies against the hCG-binding protein in the presence of goat anti-rabbit IgG. Similarly, the LH/CG receptor solubilized with 1% triton X-100 from the rat testes was also immunoprecipitated by the antibodies. Furthermore, the antibodies did not inhibit the binding of 125I iodo hCG to the hCG-binding protein or to the solubilized rat testicular LH/CG receptor. From these results it can be concluded that there exists the sharing of antigenic determinants between the hCG-binding protein from Pseudomonas maltophilia and the LH/CG receptor from the rat testes and that the antibodies specifically recognize the antigenic determinants outside the hCG-binding sites on the rat testicular LH/CG receptor. Therefore, the antibodies against the hCG-binding protein are of potential applications to studies on structure and function of the mammalian LH/CG receptors.