Histologic and cytomorphologic features of mammary carcinoma have been correlated with estrogen receptor (ER) levels determined by the dextran-coated charcoal (Scatchard) analysis in 51 primary mammary carcinomas. The results were expressed as follows: ER-positive above and ER-negative below 10 fmol/mg protein and ER-rich above and ER-poor below 250 fmol/g tissue for premenopausal patients (750 fmol for postmenopausal patients). Most lobular carcinomas were ER-positive and ER-rich (84.6%). A similarly high percentage (88.9%) of ER-positive and ER-rich determinations was seen in ductal carcinomas with tubular features, whereas only about half of the remaining ductal tumors were ER-positive or ER-rich. The highest ER values were obtained in the lobular carcinomas and in ductal carcinomas with tubular features. Three cytomorphologic indices independent of variation in tumor histology showed a strong correlation with ER values: maximal epithelial cellularity of the tumor (P less than 0.001); cellular size (P less than 0.05); nuclear size (P less than 0.05).