Aspergillus niger var. macrosporus produces two kinds of extracellular acid proteinases, i.e., proteinase B (aspergillopepsin I or proctase B) and proteinase A (aspergillopepsin II or proctase A).1 Proteinase B is a typical pepsin-type aspartic proteinase, whereas proteinase A is a non-pepsin type acid proteinase rather insensitive to specific inhibitors of aspartic proteinases, including pepstatin, DAN and EPNP.2 Proteinase A has a molecular mass of 22,265 dalton, and is composed of two peptide chains, namely Lchain (39 residues) and Hchain (173 residues), which are noncovalently bound to each other.3,4 It has no similarity in amino acid sequence with ordinary pepsin-type aspartic proteinases and appears to have fairly different substrate specificity. The active site residues have not been identified in the previous studies, although the results of chemical modification as well as its pH-activity profile indicated that certain carboxyl groups are involved in the activity.