Preparation of pregnant mare serum gonadotropin. (PMSG) was injected with the Freund's adjuvant into a rabbit, and induced the production of an anti-PMSG which showed precipitin and agglutinin properties. By means of this precipitin test, the anti-PMSG serum may be used for immunological test for pregnancy. The antiserum also has antihormonal properties against the pregnant mare serum gonadotropin activity of the serum of pregnant mare.