From hydrolysis experiments carried out on αs1-caseins A and F at pH 5·2 in the presence of 30 g NaCl/l, i.e. the conditions encountered in many young goats' cheeses, it was found that minima of 19 and 9 bonds were sensitive to chymosin in variants A and F respectively. Variant A was hydrolysed faster than variant F and the proteolytic pattern (reversed-phase HPLC and polyacrylamide agarose gel electrophoresis) differed between the variants. Hydrolysates from both variants had a number of cleavage sites in common (Leu20–Leu21, Phe23–Ala24 and Phe32–Arg33 in both variants, Leu101–Lys102 and Leu64–Lys65, Leu120–His121 and Leu83–His84, Leu142–Ala143 and Leu105–Ala106, Leu149–Phe150 and Leu112–Phe113, Leu156–Asp157 and Leu119–Asp120, Trp164–Tyr165 and Trp127–Tyr128 in variants A and F respectively), while other bonds were split only in variant A (Leu16–Asn17, Glu18–Asn19, Phe28–Pro29, Ile44–Gly45, Tyr80–Ile81, Gln82–Lys83, Tyr91–Leu92, Tyr94–Leu95, Leu109–Glu110 and Phe179–Ser180). Major cleavage sites appeared to be at Phe23–Val24, Leu142–Ala143 and Trp164–Tyr165 for variant A, and Phe23–Val24 and Leu64–Lys65 for variant F. Cleavage site Phe23–Val24 could be the origin of the first breakdown product from goat αs1-caseins A and F visible in polyacrylamide agarose gel electrophoresis.