Estrogen and progesterone receptor determination in breast cancer is of significance in determining a patient's potential response to hormonal manipulation. One practical limitation to this procedure has been availability of material for receptor assay. We describe 12 patients in whom quantitative estrogen and/or progesterone receptor analysis was accomplished utilizing tumor cells obtained from malignant effusions. The ability to obtain quantitative steroid receptor data on material derived from malignant effusions provides an effective source for this important biochemical parameter in patients in whom no other tumor tissue is available for study.