Progesterone levels in skim milk sampled from 17 Pregnant and 39 non-pregnant cows three times weekly for 4 to 9 weeks after insemination were measured by an enzyme immunoassay. The lengths of estrous cycle in non-pregnant cows were 19 days in 8, 21 days in 14, 22 days in 1, 23 days in 3, 26 days in 3, and 30 to 60 days in 10 cows, respectively. A significantly lower (p<0.01) progesterone level of skim milk in non-pregnant cows, as compared with the level in the pregnants, was observed only on the day of estrus. The milk progesterone contents 2 days before and after the estrus were high enough to indicate the active corpus luteum function. In the 10 cows which returned to estrus 30 to 60 days after insemination, an occurrence of the embryonic death after 20-22 days after the serivce was suspected. The variation of the length of the estrous cycle as well as the occurrence of embryonic mortality could be most probable cause of the false positive diagnosis of pregnancy when the milk progesterone test was applied on the 20th or 21st day after insemination.