A spontaneous mutation causing a nuclear opacity of the lens of the eye was detected among (101 X C3H) F1 hybrid mice. The nuclear opacity, provisional gene symbol Nop, is inherited as a single autosomal dominant gene. Penetrance on the genetic background of the 101-strain is complete. Heterozygotes and homozygotes are viable and fertile. The amount of protein after centrifugation at 3000 g is reduced in the cataractous lens. After isoelectric focusing a band at pH 8.5 in the protein pattern is missing. The glutathione redox-state of the cataractous lens is also affected. The amount of oxidized glutathione relative to the total amount of glutathione is increased from 2.7 to 7.8% in the Nop/ + mutant (P less than 0.01). Enzyme activities connected with the glutathione redox-cycle (glutathione peroxidase, glutathione reductase, glucose-6-phosphate dehydrogenase) are not affected. The activities of some glycolytic enzymes, phosphoglycerate kinase, glyceraldehyde-phosphate dehydrogenase, phosphoglyceromutase and triosephosphate isomerase are reduced (P less than 0.05). However, the concentration of ATP in the cataractous lens is unchanged.