D-Amino acid oxidase activity in the kidney homogenates of mice of seven strains was measured to search for a mutant for this enzyme. There was a consistent sex difference in the enzyme activity in these strains: male mice showed higher levels of the enzyme activity than females. In contrast to other strains, some mice of the ddY strain did not possess enzyme activity. This trait was inheritable, and a mouse stock without enzyme activity (DAO-) was established. The allele (Dao-1c) carried by the DAO- mice was recessive and behaved as a single autosomal gene in inheritance. Heterozygous mice for this gene (Dao-1+/Dao-1c) showed nearly half the enzyme activity of the wild-type homozygotes (Dao-1+/Dao-1+), suggesting that Dao-1c is a null allele and that there is a gene dosage effect on the enzyme activity.