The intrauterine method of assay for progesterone, of McGinty, Anderson and McCullough, was utilized for testing the blood of pregnant [female][female]. Ethereal extracts of 0.4-1.0 cc. of pregnancy serum produced positive progestational action (Corner-Allen scale). 20 samples of the 21 sera tested proved positive for progestin in at least one of the dilutions of the extracts. The reactions were all minimal and indicated that the extracts of 1 cc. of serum or less probably contained less than 0.13 [gamma] of progestationally active substance.