PIP: A clinical study to quantify estrogen receptors in tissues from malignant and benign breast tumors was conducted on 61 patients in the hospital "La Paz" of Madrid, Spain. 42 tumors were malignant, and 19 benign. The McGuire technique, based on the property of receptor protein to combine with tritiated estradiol-17 B, was used. Among the 42 biopsies with malignant histology 25 had significant quantities of estrogen receptors. Among the 19 biopsies with benign histology, only 3 had positive receptors. Endocrine treatment for such cases usually consists in administration of antiestrogens. The article also contains a brief review of the latest published literature on the subject.