Response to several types of endocrine therapy or chemotherapy was evaluated in 60 patients with breast cancer. Estrogen and progesterone receptors were determined by radioimmunoassay. Response to endocrine therapy was significantly higher (P less than 0.01) among estrogen receptor (ER)-positive cases than among ER-negative cases. The response to chemotherapy did not differ significantly between the two groups. The results of this small series support the conclusion that determination of ER is valuable in planning endocrine treatment of the breast cancer patient, whereas response to chemotherapy does not correlate with ER levels.