Mineralocorticoid receptors (MR) bind both mineralocorticoids and glucocorticoids. They are expressed in multiple tissues and mediate diverse functions. Less is known about MR regulation and function compared with other major steroid receptors, although its importance has become increasingly apparent. A significant obstacle to such studies has been the dearth of specific high-affinity MR antibodies. We have produced monoclonal antibodies against 10 different peptide conjugates, six from the N terminus (A/B domain) and four from the C terminus (steroid binding domain), with the anticipation that their individual affinities for the MR would differ depending upon its conformation, which in turn, is dependent upon the location of the receptor within the cell and the proteins associated with it. Hybridoma clones with high titers to the cognate peptide ELISA were analyzed by Western blots using protein from Chinese hamster ovary cells transfected with enhanced green fluorescent protein-rat MR cDNA and from hippocampal cytosol from adrenalectomized rats. Immunohistochemistry was done on kidney, heart, colon, and brain. Antibodies that proved to be most useful for Western blot analysis and immunohistochemistry include those raised against peptides comprising amino acids 1-18, 64-82, 79-97, and 365-381. The intensity of immunoreactivity in the cytosol compared with nucleus in the same cells differed between antibodies, suggesting that certain receptor epitopes were more or less exposed depending on the location of the receptor within the cell. In summary, several antibodies are described that recognize different parts of the MR that should facilitate the study of this important mediator of two classes of steroid hormone action.