AMH is a TGFβ superfamily member that has been used extensively as a blood marker of ovarian status in women.  AMH is synthesised as a proprotein precursor (proAMH), which can be cleaved to yield N-terminal (AMH_N), and C-terminal (AMH_C) fragments, that can complex non-covalently (AMH_N,C).  AMH_C or AMH_N,C both bind to AMH receptors while proAMH does not, and is incapable of initiating signal transduction.  Unlike most other TGFβ superfamily members, proAMH is rarely cleaved to completion when produced in cell or organ cultures.  Despite this, the molecular form of AMH in blood has not been verified.  We report here, that human blood is a mixture of proAMH and AMH_N,C.  The AMH in the blood of boys, men and premenopausal women was immunoprecipitated using antibodies to the N- and C-terminal peptides.  The N-terminal antibody precipitated bands consistent with proAMH and AMH_N which were verified with recombinant forms AMH.  This antibody also coimmunoprecipitated AMH_C from all participants.  Antibodies specific to AMH_C could precipitate recombinant AMH_C but not when AMH_N was present.  This antibody did not precipitate AMH_C from human blood suggesting that all AMH_C in human blood is bound to AMH_N.  In conclusion, AMH in blood is a mixture of two species, only one of which can bind to the AMH receptors.  The endocrine roles of AMH may be affected by signalling that is partly controlled by cleavage in the target organ.  The presence of two distinct species of AMH in blood may confound the use of AMH in clinical diagnosis.