Estrogens regulate normal sexual and reproductive development in females. Their actions are mediated mainly by estrogen receptor (ER)α and ERβ. Understanding the function of ERs necessitates knowing their cellular location and protein partners, which, in turn, requires reliable and specific antibodies. Several antibodies are available for ERα; however, discrepancies in immunoreactivity have been reported for ERβ. Here, we have developed antisera for mouse ERβ (mERβ) using a specific C-terminal 18-amino acid peptide conjugated to mariculture keyhole limpet hemocyanin. Sprague Dawley rats were immunized, and the resulting antisera were characterized by Western blot analysis of nuclear extracts from tissues of wild-type (WT) mice, and mice genetically modified to lack either ERα (CERαKO) or ERβ (CERβKO). An approximately 56-kDa protein was detected in the hypothalamus, uterus, ovary, mammary gland, testes, and epididymis of WT mice, consistent with the predicted molecular size of ERβ. In addition, the same protein band was identified in in vitro synthesized mERβ protein and in the mammary glands of CERαKO mice. The approximately 56-kDa protein was not observed in in vitro synthesized mERα protein or in any tissue examined in the CERβKO mice. Immunohistochemistry using the antisera revealed ERβ staining in the granulosa cells of WT ovaries and in the mediobasal hypothalamus, paraventricular nucleus, and cerebral cortex in the WT adult mouse brain. These data suggest that the novel rat anti-mERβ sera are specific to ERβ to allow investigators to explore to cellular and physiological role of ERβ in the brain and other mouse tissues.