The cow milk was collected with filter paper treated with ethanol during eastrus-day (0 day) and 22th and 24th day after mating. Then it was dried and stored in room temprature until analysis for progesterone by means of radioimmunoassay. The sensitivity is 13.62 pg/bule (n = 4), the coefficients of variation within a group and between groups are 8.8% (n = 10) and 16.65% (n = 8) respectively, and the recovery is 91.23% (n = 4). The average progesterone level for 22th and 24th day in the pregnant cows (6.28 +- 1.28 ng/ml) was much higher than that in the non-pregnant cow (2.00 +- 1.18 ng/ml), the difference being significant (P < 0.001). The judgement based on progesterone level (5 pregnant and 5 non-pregnant cows) faily agreed with the clinical diagnosis.