The T5P mutation in human gamma C-crystallin produces a lens cataract. Here, we have investigated the effects of the T5P mutation upon the aggregation of gamma C-crystallin in vitro and in transfected cells. By sedimentation assay and sucrose gradient centrifugation, the mutation significantly increased the aggregation of the protein and reduced dramatically its solubility in vitro. Similar effects were seen when T5P gamma C-crystallin was transfected into tissue culture cells, resulting in the formation of cytoplasmic aggregates of T5P gamma C-crystallin. Interestingly, the major lenticular protein chaperones, alpha A- and alpha B-crystallin, increased the solubility of the T5P gamma C-crystallin both in vitro and in transfected cells. More importantly, the size of the T5P gamma C-crystallin aggregates were also significantly reduced in the presence of the lenticular chaperones. These data therefore suggest a dual role for these chaperones in maintaining transparency in the lens. The first is that these protein chaperones increase the proportion of the soluble T5P gamma C-crystallin and the second is that they also reduce light scatter by reducing the aggregate size of T5P gamma C-crystallin. Both activities could modify the cataract phenotype and help explain the observed variability reported for identical gamma-crystallin mutations, which identify cataract as a polygenic disease.