Solutions of hCG and two preparations of hCGβ subunits were bioassayed after admixing with either normal rabbit serum or an anti–hCGα subunit serum (A/S). Immunoprecipitation studies with labeled antigens showed that the A/S bound hCG, but not hCGβ, at the concentration effective in neutralizing the biologic activity of both substances. Relative potency of hCGβ to 2nd I.S. hCG was measured using parallel line graded dose response assays based upon ventral prostate weights of hypophysectomized immature male rats. Biologic activity of the β subunit preparations was 174 and 269 IU/mg, but after reacting with 0.1 ml of A/S, the relative potency of the two preparations was in vivo bioassays. (Endocrinology91: 144, 1972)