Exposure of MCF-7 cells to single and/or repeated low β₯-ray doses (0.5 to 8 Gy) resulted in a decrease in the capacity of these cells to concentrate tritiated estradiol ([ 3 H]E 2 ) (reduction of the number of binding sites). The decrease in the [ 3 H]E 2 -binding capacity was higher than the survival rate, indicating that it could not be ascribed to cell death. Moreover, such low irradiation doses failed to similarly affect the specific incorporation of [ 3 H]ORG 2058, even when the progesterone receptor was induced by E 2 , a finding that rejects the hypothesis of a nonspecific effect on all steroid hormone receptors. This loss of [ 3 H]E 2 binding was reflected by the elimination of the estrogen receptor alpha (ER) when the latter was assessed by immunocytochemistry. However, additional immunochemical studies (Western blot data) performed on cell extracts under denaturing conditions failed to show any similar elimination of the ER peptide, suggesting that the loss of E 2 -binding capacity would be relevant to subtle changes in the ER structure and/or ER-associated proteins. The loss of binding capacity, produced by a 3-Gy irradiation, failed to decrease the sensitivity of the cells to E 2 , since progesterone receptor induction and growth stimulation were maintained. Insufficient ER diminution may explain this observation.