Free natural estrogens in raw and commercial whole milk were quantitated by radioimmunoassay. The ranges of concentration of estrone, estradiol 17-beta, and estriol were 34 to 55, 4 to 14, and 9 to 31 pg/ml. Proportions of active estrogens (estrone and estradiol) in the fat phases of milk by radioactive tracer on separated milk were 80% and 65%. These findings were supported by radioimmunoassay of skim milk and butter. Equilibrium dialysis of skim milk with hydrogen 3 labeled estrogens showed that 84 to 85% of estrone and estradiol and 61 to 66% of estriol were protein bound. Whey proteins demonstrated a greater binding capacity than casein. This result was confirmed by radioimmunoassay of dry curd cottage cheese and whey. The concentrations in curd were 35, 11, and 6 pg/g. In whey they were 4, 2, and 3 pg/ml. The quantity of active estrogens in dairy products is too low to demonstrate biological activity. Butter was highest with concentrations of 539, 82, and 87 pg/g. Human colostrum demonstrated a maximum concentration of 4 to 5 ng/ml for estrone and estriol and about .5 ng/ml for estradiol. By the 5th day postpartum, they decreased to become similar to cow's milk.