The formation of sugar-cataracts has been hypothesized to involve the nonenzymatic glycosylation, sulfhydryl oxidation, and aggregation of lens proteins. Cataractous lenses of diabetic and galactosemic rats were analyzed for glycosylated lysine residues in crystallins. A five- and a ten-fold increase in glycosylated lysine residues was measured in galactose and diabetic cataracts, respectively. The modification was predominant in the insoluble fraction of the lens homogenate. The proteins were further examined for the presence of disulfide bonds and high molecular weight aggregates. After careful disruption of the lens in a nitrogen environment, a cloudy solution was obtained from cataractous lenses whereas a clear solution was obtained from normal lenses. The absorbance at 550 nm of the solution of both the galactosemic and the diabetic cataracts could be decreased by approximately 50% with the addition of dithioerythritol (50 mM). The presence of high molecular weight aggregates was ascertained by sucrose gradient centrifugation and gel filtration chromatography. The proteins were heterogenous in size and showed a mol wt range of 36 to greater than 176 million daltons. Treatment with dithioerythritol induced a marked decrease in the amount of high molecular weight proteins. These data suggest that sugar cataracts of experimental animals have, in common with human cataracts, the presence of high molecular weight aggregates which are in part linked by disulfide bonds.