Anti-Müllerian hormone (AMH) is a member of the transforming growth factor-beta superfamily, which plays an important role in both ovarian primordial follicle recruitment and dominant follicle selection in mice. However, the role of AMH in folliculogenesis in humans has not been investigated in detail. In the present study, AMH expression was assessed using immunohistochemistry in ovarian sections, obtained from healthy regularly cycling women. To this end, a novel monoclonal antibody to human AMH was developed. AMH expression was not observed in primordial follicles, whereas 74% of the primary follicles showed at least a weak signal in the granulosa cells. The highest level of AMH expression was present in the granulosa cells of secondary, preantral and small antral follicles <or=4 mm in diameter. In larger (4-8 mm) antral follicles, AMH expression gradually disappeared. In conclusion, in the human AMH expression follows a similar pattern as compared to the mouse and rat, suggesting an important role of AMH in folliculogenesis.