The presence of estrogen receptors (ER) has been studied in 70 patients with primary operable breast cancer. The tumors were graded histologically according to Bloom and Richardson criteria. A significant correlation was found between these two prognostic factors: the better--differentiated tumors (I + II histological grade) had significantly higher level of ER than that in the poor--differentiated tumors. Estrogen receptor content of primary breast cancer as a function of tumor grade can be useful in selection of the most efficient treatment modalities for individual patients.