This stduy was carried out to evaluate the ability of clinical application of pregnancy diagnosis based upon the determination of progesterone in serum, utilizing EIA-kit of progesterone concentrations in the serum were assayed by radioimmunoassay. 1. The progesterone concentrations of the pregnant cows(2.40$pm$0.34ng/ml) were significantly higher than those of non-pregnant cows(1.03$pm$0.09ng/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 EIA-kit of progesterone were 95.0% for non-pregnant cows, and 92.3% for pregnant cows. 3. During 20 to 22 days after artificial insemination, the accuracy of pregnancy diagnosis from serum progesterone concentrations were 100% for non-pregnant cows(<1.4ng/ml), and 96.2% for pregnant cows( 2.0ng/ml). The average overall accuracy of prediction for pregnant and non-pregnant cows were 98.1%. 4. Accordingly, the pregnancy diagnosis from EIA-kit of progesterone is thought to be recommendable because this early diagnostic means are simple with accurate results.