This experiment was carried out to determine the progesterone and oestradiol level in milk during the estrous cycle of dairy cows by radioimmunoassay and to apply it to early pregnancy diagnosis and control of infertility. Whole milk samples were collected at afternoon milking from 61 cows in the Korea-Germany Dairy Farm, National Agricultural Cooperative Federation in An-Seong. Nine cows were sampled 5 times at intervals of 5days during a single estrous cycle to establish the pattern of progesterone and oestradiol levels and 52 cows were sampled twice on day of estrus and day 23 to 26 after artificial insemination to diagnose the pregnancy and infertility. 1. The progesterone levels in milk during a single estrous cycle were the highest on day 15 with a mean of 3.2±0.4 ng/㎖ and were the lowest on day 0 and next estrus. The levels of progesterone on day 5 and day 10 Here found especially to be variable among individuals. There were no significant differences in progesterone levels between the length of estrous cycle. The oestradiol levels in milk were raised at the estrus with a mean of 34.5 to 37.0pg/㎖ and the levels of oestradiol during the luteal phase (day 5 to day 15) were not variable at level of 17.5 to 19.2pg/㎖. 2. The progesterone level, 4.2±0.5ng/㎖, of pregnant cows on day 23 to 26 after insemination was significantly higher than 1.6±0.9ng/㎖ of non-pregnant cows on same days. These levels in milk were significantly higher in pregnant cows that showed the first estrus within 51 to 80 days after calving and conceived within 60 to 90 days after calving and although there were no significant differences, the levels progesterone in milk were also higher in pregnant cows that showed the conception at the 1st insemination and was within the 3rd calving.