From early embryonic life, anti-Müllerian hormone (AMH) is produced by Sertoli cells and is essential for male sex differentiation. In females, AMH is produced by immature granulosa cells (GCs) but a definitive function in females is uncertain. We have assessed the cellular localization and specificity of a panel of five novel high-affinity AMH monoclonal antibodies. Two recognize the mature C-terminal form of AMH, whereas three recognize the active pro-mature form of AMH in human tissue. The antibodies were tested on fetal male testicular and mesonephric tissue aged 8-19 weeks post conception (pc), fetal male serum aged 16-26 weeks pc and human immature GCs by immunofluorescence, immunohistochemistry, ELISA and western blotting. The active pro-mature forms of AMH were expressed in both Sertoli cells from human fetal testis and human immature GCs. In contrast, the mature C-terminal form of AMH was hardly detected in Sertoli cells, but was readily detected in GCs. This particular form was also located to the nucleus in GCs, whereas the other investigated AMH forms remained in the cytoplasm. Interestingly, the distribution of the AMH forms in the fetal serum of boys showed that the fraction of inactive precursor AMH only accounted for 4.5% ± 0.6 (mean ± SD) of the total AMH measured, and the remaining AMH was the active pro-mature form. Furthermore, western blot analysis demonstrated a number of previously unrecognized molecular forms of AMH. The present findings suggest that processing of AMH is a tightly regulated process, which is likely to be important for the function of AMH and which differs between the two sexes.