The presence of estrogen receptors (ER) was determined in 111 human breast cancer specimens. In 61% of the tumors, specific estrogen binding was found and in 39% of the tumors ER was absent. In 69 tumors no correlation was found between the histological grading of the tumor and the level of ER. The values of ER in tumors from patients over 50 years of age were usually much higher than those for patients under 50 years of age. Different methods of endocrine therapy were applied in 20 patients. In 10 of 15 patients with ER positive tumors, endocrine therapy resulted in remission. Only 1 of 5 patients with ER negative tumors responded with remission. It is concluded that estimation of ER in tumor tissue is helpful in the selection of patients for endocrine therapy.