A new cataract model originated in a recombinant inbred (RI) strain, CXS4 or CXSD (D), between BALB/cHeA(BALB/c or C) and STS/A(STS or S) mice. Opacity appeared as a white pinpoint focus in unpigmented eyes of albino mice from 5 weeks old. All the mice were bilaterally affected by 14 weeks old. They were fully viable and fertile. There was no sex difference in incidence of cataract. Histologically, the 3-4 months old mice showed vacuoles in the lens cortex. The vacuoles were spread all over the lens cortex in advanced cases. Ruptures of the lens nucleus to the vitreous chamber was a typical occurrence. For elucidation of the mode of inheritance, F1 hybrids (CXD and SXD) and backcross progenies [(CXD)F1XD and (SXD)F1XD] were analysed. No affected mice were observed in F1 hybrids. In backcross progenies, the segregation ratio of affected and normal mice was close to 1:1 in both matings. We conclude that the cataract is inherited by an autosomal single recessive gene. This mutant gene is provisionally named 'lens rupture 2' (gene symbol 1r2, Mouse Genome Database Accession No. MGD-JNUM-37399). The new cataract model mouse will be a good tool for the genetic analysis and molecular biological study of cataractogenesis.