Estrogen receptors (ER's) were determined by agar gel electrophoresis in 105 human mammary tumor biopsies. Fifty-three tumor specimens revealed measurable amounts of receptor, and 64% of these patients showed an objective remission after endocrine treatment. If we also include cases with partial remission, previous response, and arrested growth but no actual remission, the benefit rate increases to 87%. The corresponding percentage in the ER-negative groups is 13%. In a retrospective study of 42 patients with human breast cancer, a correlation between ER finding and remission rate after chemotherapy was found. Of the ER-negative patients, 71% showed an objective remission after polychemotherapy, whereas only 43% of the ER-positive cases revealed such remission. The presented data are discussed with special regard to their clinical relevance.