The crystallins in the lenses of ICR/f mutation rat, a known hereditary cataract model, were analyzed during cataractogenesis. Opacification of the mutant lenses was found to be accompanied by changes in crystallin structure and composition, including several deletions of the N-terminals of β-crystallins and low molecular weight α-crystallins. Because similar deletions were observed when the soluble fraction of normal lens protein was incubated with calpain, we considered that calpain could be related to the deletions in mutant lenses. Although measurement of the content of calpain protein by the ELISA method revealed no significant difference between mutant and normal lenses, it was found that the concentrations of Ca²⁺ and K⁺ were different between the two lenses and that calpain activity was dependent on both ion concentrations. Endogenous m-calpain in the soluble fraction from normal lenses was activated by addition of 1 mM calcium chloride in the presence of 50 mM KCl (the same concentration as in mutant lenses), and insoluble protein was found in the fraction 1 d after calpain activation. On the other hand, the presence of 120 mM KCl (the concentration in normal lenses) inhibited calpain activity and prevented this insolubilization. These results suggest that calpain in mutant lenses is involved in the proteolysis of crystallins and the progression of cataract formation.