These experiment was carried out to improve the precision of early pregnancy diagnosis in dairy cattle. Changes in progesterone concentration of milk were measured by Enzyme Immunoassay(EIA) in 73 cows up to 21 days after insemination. The average concentraton of progesterone in milk was 1.9ng/ml at eatrus ; it increased to 17.8ng/ml on day 14, and thereafter it declined to 4.3ng/ml on day 21 in nonpregnant cows. Whereas in pregnant animals, it was maintained and elevated further to 22.2ng/ml on day 21. The accuracy of the pregnancy diagnosis for animals classified as positive (pregnant ; over 10ng/ml and decreasing rate<1.5) and negative (non-pregnant ; under 7ng/ml and decreasing rate>1.5) were 95% and 100% respectively. The samples ranging between 7ng/ml and 10ng/ml were classified as positive (decreasing rate<1.5) and negative(>1.5), which accuracy was 54.5% and 100% respectively. However this range appears to be of the most interest for the veterinary practitioner since cows in proestrus or early interestrus tend to have milk progesterone levels within this value. Causes for the insufficient precision of false pregnancy diagnosis are discussed.