Summary.The milk prothrombin level was studied in cows and human beings. In the months before and directly after calving one can demonstrate considerable amounts of prothrombin in cow's milk. The woman colostrum also contains prothrombin but only in small amounts. At other times in the lactation period the cow's milk and human milk contain only negligible amounts of prothrombin.The prothrombin content in the cow's colostrum is of no importance to the increase of the prothrombin in the blood of the new‐born cal