Incubation of soluble proteins from rat lens with the protease calpain II caused the precipitation of β‐crystallin polypeptides. Two‐dimensional electrophoresis and sequence analysis identified β‐crystallin polypeptides both before and after their precipitation by calpain II. β‐crystallin polypeptides precipitated by calpain were cleaved at their NH2‐terminal extensions. These cleavage sites were similar to cleavage sites occurring in β‐crystallin polypeptides precipitated during formation of experimental cataract induced by an overdose of selenite. These data suggested that calpain II caused β‐crystallin insolubilization during cataract formation, and indicated that the process can be mimicked in vitro.