Summary. A highly reliable technique for the determination of progesterone in milk has been developed, and its application to pregnancy diagnosis in cows is reported. The method is characterized by an activated charcoal treatment of the samples, extraction of the absorbed progesterone by an organic solvent, followed by RIA. The application of the method to pregnancy diagnosis, 21\p=n-\23 days after artificial insemination, shows that 5 ng/ml is the most useful value to discriminate between pregnant and non-pregnant animals, with greatest precision on the 21st day, in that 93\m=.\5%of pregnant and 100% of non-pregnant animals were correctly diagnosed.