A detailed histopathological review of 140 primary breast cancers analyzed for estrogen receptor protein (ERP) was carried out and a variety of morphological features correlated with ERP results. ERP in cytosols was incubated with [3H]estradiol in the presence and absence of cold estradiol and assayed by agargel electrophoresis. Tumors binding greater than 10 fmol estradiol/mg tissue protein were classified as receptor-positive. Seventy-seven percent of the 116 infiltrating duct carcinomas were ERP-positive. The well-differentiated tumors of this group had a higher incidence of ERP-positivity than the poorly differentiated ones. The ten predominantly or exclusively intraductal carcinomas and the seven medullary tumors were less frequently positive than the main group of infiltrating ductal cancers. The three colloid, two tubular, and two lobular carcinomas in this series were all ERP-positive. When receptor measurements are evaluated, consideration should be given to the degree of differentiation and the histological type of tumor, in addition to other factors known to influence receptor levels such as menopausal status and seasonal variation.