A total of 219 specific-locus, 35 dominant cataract and 44 enzyme-activity mutations induced in spermatogonia of mice by radiation or ethylnitrosourea (ENU) treatment were characterized for homozygous viability as well as fitness effects on heterozygous carriers. For all 3 genetic endpoints, the frequency of homozygous lethal mutations was higher in the group of radiation-induced mutations than in the ENU-treatment group. These observations are consistent with the hypothesis that radiation-induced mutations recovered in the mouse are mainly due to small deletions while ENU induces mainly intragenic mutations. The overall fitness of mutant heterozygotes was reduced for the group of radiation-induced specific-locus, dominant cataract and enzyme-activity mutations while the ENU-induced mutations exhibited no reduction in fitness. The fitness reduction of heterozygous carriers for a newly occurring mutation in a population is important in determining the persistence of the mutation in a population, and thus the total number of individuals affected before a mutation is eventually eliminated from the population. For the present results a maximal persistence of 12 generations and a minimal persistence of 3 generations is estimated. These results are consistent with the 6-7-generation persistence time assumed by UNSCEAR (1982) in an estimate of the overall effects of radiation-induced mutations in man.