C-Terminal truncated α-crystallins have been found in lenses of hereditary cataractous rat ICR/f, including two truncated αB-crystallins and several truncated αA-crystallins. These truncated crystallins probably resulted from degradation by m-calpain and Lp82. The αB-crystallin with five amino acid residues deleted showed decreased chaperone activity. Compared with α-crystallins from the normal rat lenses, overall chaperone activity of α-crystallins from the mutant lenses, including the above truncated αB-crystallin, was remarkably reduced. The decreased chaperone activity accompanying the increase in C-terminal truncated α-crystallins may cause the insolubilization of many proteins in the mutant lenses, which it is likely to lead to the progression of cataract formation.