Specific polyclonal antisera to the rat estrogen receptor (rER) were developed using a synthetic peptide as the antigen. The peptide corresponds to amino acids 270-284 deduced from the cloned rER cDNA and has no homology to other steroid hormone receptors. Antipeptide antisera raised in rabbits specifically recognize a 67,000 mol wt protein, shown to be the rER, in Western blot experiments. In immunoprecipitation experiments, one tested antiserum bound unoccupied as well as 17 beta-estradiol-occupied rERs, indicating that this region is exposed in both receptor forms. This antiserum shows no cross-reactivity with rat progesterone or glucocorticoid receptors. Cross-reactivity with rabbit, human, and, to a lesser extent, bovine ERs was observed.