Angiotensin I-converting enzyme (ACE) inhibitory activities in untreated koumiss and koumiss digested with ACE, pepsin, trypsinase, and chymotrypsin were compared and analyzed. Four novel ACE inhibitory peptides (P(I), P(K), P(M), and P(P)) were purified using ultrafiltration and high performance liquid chromatography (HPLC). The classification study showed that these 4 peptides were of the true inhibitor type. The amino acid sequences of these peptides are YQDPRLGPTGELDPATQPIVAVHNPVIV, PKDLREN, LLLAHLL, and NHRNRMMDHVH, respectively. Their individual IC(50) (50% inhibitory concentration) values were as follows: 14.53+/-0.21microM, 9.82+/-0.37microM, 5.19+/-0.18microM, and 13.42+/-0.17microM. From sequence analysis, we determined that P(I) was part of beta-casein in mare's milk. The 3 peptides P(K), P(M), and P(P) did not correspond with any known milk protein. The results suggest that koumiss is rich in ACE inhibitory peptides, and the ACE inhibitors in koumiss are of the pro-drug type or a mixture of the pro-drug type and the true inhibitor type. These results may provide evidence about the beneficial effects of koumiss, especially on cardiovascular health.