Soybean proteins, beta-conglycinin and glycinin were hydrolysed by an acid proteinase from Monascus purpureus. The degree of hydrolysis and inhibitory activities of angiotensin I-converting enzyme (ACE) increased with increasing proteolysis time. After 10 h of incubation, the IC50 values of the beta-conglycinin and glycinin hydrolysates were determined, as 0.126 mg/ml and 0.148 mg/ml, respectively. Four ACE inhibitory peptides were isolated from the soybean protein hydrolysates and identified by protein sequencer. ACE inhibitory peptides isolated from the P-conglycinin hydrolysate were identified as LAIPVNKP (IC50 = 70 mu M) and LPHF (670 mu M), and those from the glycinin hydrolysate as SPYP (850 mu M) and WL (65 mu M). The inhibitory activity of SPYP markedly increased after successive digestion by pepsin, chymotrypsin and trypsin in vitro. (c) 2004 Elsevier Ltd. All rights reserved.