For determination of early pregnancy in cows milk samples were drawn under field conditions on the 20th day after insemination from 168 animals and mailed to the laboratory for progesterone analysis. Values above 11 ng/ml were considered indicative of pregnancy, and those below 2 ng were classified non-pregnant. Values ranging between 2 and 11 ng/ml were graded according to the progesterone and fat concentration in milk samples. The method was based on results obtained in earlier studies, which showed a highly positive correlation between changes in progesterone concentrations and changes of milk fat concentrations when an active corpus luteum is present. A diagnosis of pregnant was in agreement with results derived from a clinical pregnancy diagnosis by palpation performed 8 to 9 weeks after insemination in 86% of cases; in all cases of a negative diagnosis the cows were found nonpregnant.