From hydrolysis experiments carried out on α_s1-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 α_s1-caseins A and F visible in polyacrylamide agarose gel electrophoresis.