The technique of biomicroscopic eye examination followed by breeding tests provides a new method for detection of dominant mutations in mice. A total of 11 cataract mutations were found among 17,436 offspring of irradiated male mice. No cataract mutations were observed in 8,174 offspring from the control group. All mutations were phenotypically different. Of the 11 cataract mutations, 3 were semilethal in heterozygous condition, 7 were lethal in homozygous condition, and one presumed mutant was sterile. Seven mutations had complete penetrance whereas penetrance of three mutations was reduced. The rate of dominant mutations affecting an organ system in mice is of main importance for the quantification of the overall genetic damage due to dominant mutations in man.