Estrogens and androgens are essential for the maturation of the ovarian follicle and normal fertility in the female. We have used antibodies specific for both forms of estrogen receptor (alpha [ERα] and beta [ERβ]) and androgen receptor (AR) to investigate the pattern of receptor expression in ovaries obtained from women and from a New World primate, the Common marmoset (Callthrix jacchus). On Western blots, three antibodies directed against different peptides within human ERβ all recognized recombinant human (h) ERβ but did not bind to recombinant hERα. The ERβ protein was extracted from human ovary and prostate and marmoset ovary. In marmoset and human ovaries, ERβ protein was detected in the nuclei of granulosa cells in all sizes of follicle (both before and after formation of the antrum), and it was also detected in thecal cells, corpora lutea, surface epithelium, and stroma. In contrast, ERα protein was not detected in the nuclei of granulosa cells in preantral follicles, was low/absent from stromal and thecal cells, but was expressed in granulosa cells of antral follicles and in the surface epithelium. The pattern of expression of AR protein more closely resembled that of ERβ than ERα. In conclusion, three independent antibodies have demonstrated convincingly that ERβ is expressed in a wide range of cells in the primate ovary. Granulosa cells in preantral follicles could contain ERβ:β dimers. In antral follicles, however, ERα is also expressed, and the formation of homo- or heterodimers containing ERα may influence the pattern of gene activation within these cells.