Bovine lens calpain (Ca2+-dependent cysteine proteinase; EC 3.4.22.17) was shown to catalyze limited proteolysis of A and B chains of alpha-crystallin in vitro. The sites of cleavages were determined by isolating and analyzing the peptide fragments formed using several different methods, including high-performance liquid chromatography, cyanogen bromide cleavage, and carboxypeptidase digestion. The results indicated that calpain cleaved both A and B chains at their respective carboxyl-terminal regions. A chain was cleaved at A(Arg163-Glu164) bond and A(Ser162-Arg163) bond, the former being split with 2.4 times preference over the latter. B chain was cleaved at B(Thr170-Ala171) bond and B (Arg163-Glu164) bond, the former being preferred 6.5 times. Peptide cleavage at any other sites were not detected by the present method of analysis.