AbstractAntiserum to human chorionic gonadotropin (anti‐HCG) with measured capacity to inhibit its antigen was used in conjunction with HCG in intact immature and in hypophysectomized female rats to determine if this hormone has intrinsic follicle stimulating properties. In intact rats, 100, 200, 500, 1000, 1500, 2000 and 2500 I.U. of HCG were given alone and in conjunction with anti‐HCG. In hypophysectomized rats, 400, 800, 1600, 3200, and 6400 I.U. of HCG were given alone and with anti‐HCG. Anti‐HCG inhibited both the second rise in ovarian weight in intact females and follicular development in hypophysectomized females in a manner anticipated on the basis that these responses depended upon the HCG molecule. In hypophysectomized females inhibitition of follicular growth preceded inhibition of interstitial cell development. This result is also in accord with the view that both of these responses depend upon the HCG molecule.The anti‐HCG serum did not inhibit synergism between HCG and NIH‐FSH‐S1, an ovine pituitary FSH, with doses of 100 and 200 I.U. of HCG combined with 240 m̈g of FSH. An amount of antiserum which would inhibit 500 I.U. of HCG augmented rather than inhibited the response to 1000 μg of NIH‐FSH‐S1 (43 mg ovarian weight as compared to 29 mg).We conclude from these data that the HCG molecule has intrinsic follicle sti