αA-crystallin (αA) and αB-crystallin (αB) are among the predominant proteins of the vertebrate eye lens. In vitro, the α-crystallins, which are isolated together as a high molecular mass aggregate, exhibit a number of properties, the most interesting of which is their ability to function as molecular chaperones for other proteins. Here we begin to examine the in vivo functions of α-crystallin by generating mice with a targeted disruption of the αA gene. Mice that are homozygous for the disrupted allele produce no detectable αA in their lenses, based on protein gel electrophoresis and immunoblot analysis. Initially, the αA-deficient lenses appear structurally normal, but they are smaller than the lenses of wild-type littermates. αA^−/− lenses develop an opacification that starts in the nucleus and progresses to a general opacification with age. Light and transmission electron microscopy reveal the presence of dense inclusion bodies in the central lens fiber cells. The inclusions react strongly with antibodies to αB but not significantly with antibodies to β- or γ-crystallins. In addition, immunoblot analyses demonstrate that a significant portion of the αB in αA^−/− lenses shifts into the insoluble fraction. These studies suggest that αA is essential for maintaining lens transparency, possibly by ensuring that αB or proteins closely associated with this small heat shock protein remain soluble.