The determination of estrogen receptor (estrophilin) in human breast cancers, both primary and metastatic, can furnish information useful to the clinician in his choice of the optimal therapy for the individual patient with advanced disease. Of patients with significant tumor estrophilin levels, most, but not all, will respond favorably to endocrine therapy. Women whose cancers lack sufficient amounts of estrophilin have little or no chance of benefit from endocrine ablation or hormone administration and probably should be treated directly by alternative types of therapy.