Breast cancer is the result of a multistage carcinogenic process. Initiation, promotion, dependency and autonomy make up a sequence of experimentally distinguishable phases of this process. Progression--the transition from dependency on hormonal support to autonomy--is demonstrable clinically. High-affinity saturatable estrogen binding by breast cancer cytosols distinguishes endocrine-responsive mammary neoplasms from autonomous breast cancers. Approximately 70% of neoplasms containing estrogen-recepor protein at a level of 2.5 femtomoles per mg. protein or higher regress after endocrine ablation (ovariectomy in premenopausal women; adrenalectomy or hypophysectomy in postmenopausal women). Only about 5% of neoplasms lacking the receptor will respond to these maneuvers. Estrogen-receptor content also predicts clinically for estrogen and androgen responsiveness, and experimentally for prolactin dependency. Fifty per cent of primary breast cancers in women are receptor-positive. Normal breast tissue and benign breast lesions characteristically lack receptor protein. The receptor proteins appear to be induced in neoplastic cells during mammary carcinogenesis in endocrinologic settings where non-cancerous breast cells do not contain free receptor in large amounts and fail to manifest endocrinologic growth stimulation. Implications of these findings for endocrinologic management of disseminated mammary cancer, adjuvant therapy, and breast cancer prevention are discussed.