This monoclonal rabbit antibody reacts with human estrogen receptor α (ER α). Estrogens have been found to be preferentially concentrated in the estrogen target organs of animals and in human breast cancers, and it is well documented that the mitogenic effects of estrogen are mediated by ER. Historical studies have shown that ER status is correlated with untreated outcome and with response to anti-hormonal therapy, e.g. tamoxifen (1). The antibody can be used in the detection of human estrogen receptor in tissue sections of human breast cancer by immunohistochemistry for assessment of estrogen receptor status in human breast carcinomas.