A preparation of human chorionic gonadotropin (HCG) was injected with Freund's adjuvant into rabbits, resulting in the production of an antiserum that had precipitating properties. By using the rat ventral prostate method of bio-assay, it was possible to demonstrate the antihormonal properties of the antiserum. The antiserum was used in a serologic test for pregnancy utilizing the gelagar plate technique of Ouchterlony; no false positive results were observed. The antiserum also had antihormonal properties against the chorionic gonadotropic activity of the serum of pregnant women.