PIP: A radioimmunoassay technique for determining concentrations of progesterone in the milk of cattle is described and its application as a test for pregnancy and as a diagnostic tool in therapeutic treatments is reported. The technique is of value in that it verifies nonpregnancy in animals with progesterone concentrations of less than 2 ng/ml milk. In animals with higher concentrations, error rates of 14-38% have occurred. With the additional sampling of progesterone in milk fat, the error rate for positive pregnancy can be reduced, though biological differences preclude total accuracy. The Bavarian animal Health Service offers sampling vials for pregnancy testing, estrus control, and control of therapeutic treatment.