A survey has been made of the steroids in cows' milk using a method for the group analysis of steroids. Serial studies during and after pregnancy showed changes in progesterone concentration consistent with known variations in plasma concentrations. More progesterone was obtained from samples taken at the later stages of milking and samples obtained in the evening generally contained more progesterone than morning samples. Steroids were concentrated in the cream.
Relatively constant levels of 5α-androstane-3,17-dione were found and it is suggested that this steroid is of adrenal origin. The predominance of 5α steroids in milk suggests that these are derived, at least in part, from metabolism in the mammary gland. The possible use of these findings in diagnosis of pregnancy and in studies of the hormonal control of lactogenesis is indicated.