This study was carried out to evaluate the ability of clinical application of pregnancy diagnosis based upon the determinatin of progesterone in milk, utilizing a chymosin inhibitor labelled with progesterone and monoclonal antibody to progesterone, and its compared with progesterone concentrations in the milk were assayed by radioimmunoassay. 1. The progesterone concentration of the pregnant cows (2.07$pm$0.54ng/ml) were significantly higher than those of non-pregnant cows (1.04$pm$0.19 ng/ml), and thereafter began to increase and maintained high levels. 2. During 20 to 22 days after artificial insemination, the accuracy of pregnancy diagnosis from monoclonal antigen of progesterone were 92.9% for non-pregnant cows, and 88.5% for pregnant cows. 3. During 20 to 22 days after artificial inseminatin, the accuracy of pregnancy diagnosis from milk progesterone concentrations were 92.9% for non-pregnant cows(<3.4ng/ml), and 92.3% for pregnant cows( 4.0ng/ml). The average overall accuracy of pregnancy prediction for pregnant and non-pregnant cows were 92.6%. 4. Accordingly, the pregnancy diagnosis from monoclonal antigen of progesterone is thought to be recommendable because this early diagnostic means are simple with accurate result.