The autosomal, dominant mutation Scat (suture cataract) was found in (101/El x C3H/El)F1-hybrid mice. The severity of the cataract is dependent on the gene dose. The mutation causes an anterior suture opacity in heterozygotes and microphthalmia with vascuolated lenses in homozygotes. In histological sections of lenses the heterozygotes exhibit a hydropic swelling of lens epithelium, whereas in homozygotes interruption and degeneration of lens fibers as well as clefts and folds of the capsule were observed. The mutation has a complete penetrance and constant expressivity. The body weight of the mutants is not altered: the mutation has no effects on fertility or viability. The lens wet and dry weights are diminished (more pronounced in the homozygotes). The water content of the lens is enhanced only in the homozygous Scat mutants. Biochemically, the lenticular content of water-soluble proteins is decreased in the homozygous Scat mutants. By electrophoresis, in the lenses of homozygous Scat mutants a different pattern of water-soluble proteins could be observed. The lenses of both, heterozygous and homozygous Scat mutants exhibit enhanced Na+,K+-ATPase activity and a decreased ATP concentration. The genetical, morphological or biochemical data suggest that the effect of the Scat mutation is distinct from other described cataract mutations in mice.