Our aim here was to compare a new enzyme immunoassay for estradiol receptors (ER) in breast cancer with a standard assay involving radiolabeled estradiol. A significant correlation was obtained between the standard binding assay and immunoassay (rs = 0.945, p less than 0.001, n = 100). Immunoreactive ER were also found in endometrial carcinomas but not in lung carcinomas, malignant melanomas, or basal cell carcinomas. Patients with breast cancers containing immunoreactive ER survived significantly longer after operation than patients whose tumor lacked immunoreactive ER. This immunoassay for ER is simpler than the binding assay and requires less time and tissue.