Serum of parous women and of women on oral contraceptives contains trace amounts of a protein which, in analogy to the conditions prevailing in experimental animals, is anticipated to be related to the processes preceding the formation of mammary cancer. In mice, the analogous serum protein is controlled by the endocrine system, in particular by an unknown pituitary hormone and by certain estrogens, as well as by genetic factors. Male subjects and nulliparous women who do not take oral contraceptives are usually devoid of this trace serum protein. The present investigation is intended to purify this trace serum protein and to study its chemical, physical and biological properties.