The Nakano cataract gene, nct, was introduced into Balb/c mice by repeated backcrosses to elucidate the possible effects of background genes on its expression. The resulting congenic Balb/c-nct/nct mice were characterized by retarded and sporadic cataract formation with a tendency of further retardation in males and by the different disease process of cataract as compared with Nakano mice. The age of 50% cataract incidence was 60 days in females and 90 days in males compared with 22 days in Nakano mice, and lens opacification commenced in a diffuse, mild form at the cortex in congenic but in a pin-head, intense form at the core in Nakano mice. Sex hormones seemed to be involved in the difference in cateractogenesis between male and female mice. Microphthalmia was slighter in degree in Balb/c background mice. The results indicated that the nct-dependent cataractogenesis may be influenced by background genes and some non-hereditary factors. Balb/c-nct/nct mice will provide a new type of hereditary cataract model.