The proteolytic specificity of chymosin (EC 220.127.116.11) on bovine alpha(s1)-casein at 30-degrees-C in phosphate buffer, pH 6.5 and at pH 5.2 in the presence of 5% (w/v) NaCl was investigated. Peptides (pH 4.6-soluble) were isolated by reversed-phase HPLC and identified from their amino acid sequence: the identity of some peptides was confirmed by mass spectrometry and/or amino acid composition. The small peptides produced at pH 6.5 were Arg1-Phe23, Phe24-Phe28, Phe24-Leu40(?), Phe150-Phe153, Phe150-Leu156, Tyr154-Tyr159, Tyr154-Trp164, Asp157-Trp164 and Tyr165-Trp199. The same peptides, except Tyr154 Trp164, were produced at pH 5.2 in the presence of NaCl and, in addition, the peptides Arg1-Leu11, Phe24-Phe32, Lys102-Leu142, Ala143-Leu149 and Tyr165-Phe179. The rates of production of individual peptides differed under the two conditions studied but Arg1-Phe23 and Tyr165-Trp199 were the first and second peptides produced under both conditions. Pathways are proposed to interpret the proteolysis of alpha(s1)-casein in solution under the conditions of this study.