The overall goal of this study was to provide data on the function and physiologic significance of lens calpain I, a cysteine protease requiring low amounts of calcium for activation. Reverse-transcriptase polymerase chain reaction was used to detect mRNAs for calpains I and II in young rat lenses. An in vitro model of crystallin precipitation was used to assess the ability of calpain I to induce hydrolysis and precipitation of crystallins. We found that incubation of crystallins with purified calpain I was indeed a powerful inducer of crystallin precipitation. However, mRNA levels for calpain I in whole lens appeared to be lower compared to calpain-II mRNA. Participation of calpain I in crystallin precipitation during normal maturation of rodent lenses or cataract formation is thus theoretically possible, but unlikely, because of the low level of expression of calpain I.