Recombinant human αs1-casein expressed in Escherichia coli was purified and digested with trypsin in an attempt to find peptides with angiotensin-I-converting enzyme (ACE) inhibitory activity. Three novel ACE inhibitory peptides, A-II, B-II and C, were isolated and their amino acid sequences identified as Tyr–Pro–Glu–Arg (residues 8–11), Tyr–Tyr–Pro–Gln–Ile–Met–Gln–Tyr (residues 136–143) and Asn–Asn–Val–Met–Leu–Gln–Trp (residues 164–170) respectively. ACE inhibitory activities were measured for the corresponding synthetic peptides, and the ACE IC50 (the amount of peptide causing 50% inhibition of ACE activity) values of A-II, B-II and C estimated to be 132·5, 24·8 and 41·0 μmol/l respectively. Peptides A-II and C were resistant to further digestion by pepsin, whereas peptide B-II was hydrolysed. All three peptides were resistant to digestion by chymotrypsin. These ACE inhibitory peptides may prove useful for oral administration in the treatment of hypertension.