The specific activity of erythrocyte enzymes was measured to detect gene mutations in F(1)-offspring of male mice treatment with different doses (80, 160, or 250 mg/kg body weight) of ethylnitrosourea (ENU). Altogether 13,230 offspring were screened for 10 enzyme activities. Mutants with reduced activity as well as mutants with enhanced activity were found. Of the 36 independently observed mutations, 20 were homozygous lethal. Genetic and biochemical characterizations were routinely performed. These mutants provide insight into the mechanism of ENU mutagenesis and can serve as models for structure-function studies of the corresponding enzymes.