α-Crystallin, one of the major proteins in the vertebrate eye lens, acts as a molecular chaperone, like the small heat-shock proteins, by protecting other proteins from denaturing under stress or high temperature conditions. α-Crystallin aggregation is involved in lens opacification, and high [Ca2+] has been associated with cataract formation, suggesting a role for this cation in the pathological process. We have investigated the effect of Ca2+ on the thermal stability of α-crystallin by UV and Fourier-transform infrared (FTIR) spectroscopies. In both cases, a Ca2+-induced decrease in the midpoint of the thermal transition is detected. The presence of high [Ca2+] results also in a marked decrease of its chaperone activity in an insulin-aggregation assay. Furthermore, high Ca2+ concentration decreases Cys reactivity towards a sulfhydryl reagent. The results obtained from the spectroscopic analysis, and confirmed by circular dichroism (CD) measurements, indicate that Ca2+ decreases both secondary and tertiary-quaternary structure stability of α-crystallin. This process is accompanied by partial unfolding of the protein and a clear decrease in its chaperone activity. It is concluded that Ca2+ alters the structural stability of α-crystallin, resulting in impaired chaperone function and a lower protective ability towards other lens proteins. Thus, α-crystallin aggregation facilitated by Ca2+ would play a role in the progressive loss of transparency of the eye lens in the cataractogenic process. © 2002 Elsevier SScience B.V. All rights reserved.