The mung bean trypsin inhibitor has been found to be microheterogeneous at N-terminalregion due to the presence of several isomers. After treatment with aminopeptidase M it becomeshomogeneous and is suitable for sequence determination. Based on the determination of the structures of two active fragments the complete aminoacid sequence of mung bean trypsin inhibitor has been elucidated. It consists of 72 amino acidresidues with 7 pairs of disulfide bonds. The results show that this inhibitor belongs to theBowman-Birk inhibitor family.