During the analysis of liver protein expression in the offspring of male mice irradiated with fission-spectrum neutrons, one offspring displayed a heritable 50% decrease in the abundance of two proteins. Homozygous mice lacking detectable quantities of these proteins were obtained through breeding. Characterization of this protein deficiency has identified these liver proteins as forms of the enzyme 10-formyltetrahydrofolate dehydrogenase (10-formyl-THF DH; 10-formyltetrahydrofolate:NADP+ oxidoreductase, EC 126.96.36.199). NH2-terminal sequence analysis demonstrated that both proteins share identical sequences in the first 25 residues, and this sequence matches (96% identity) that of rat and human 10-formyl-THF DH. In addition, these proteins showed cross-reactivity to polyclonal antiserum raised against purified rat 10-formyl-THF DH. Southern (DNA) blot analysis revealed a restriction fragment length polymorphism consistent with a deletion mutation in the 10-formyl-THF DH structural gene in homozygous mice. Results of Northern (RNA) blot analysis demonstrated the absence of 10-formyl-THF DH mRNA in mice lacking 10-formyl-THF DH protein. Furthermore, liver cytosolic 10-formyl-THF DH enzymatic activity was undetectable in homozygotes. Measurement of hepatic folate pools showed that in homozygotes the total folate pool is decreased and the level of tetrahydrofolate is markedly depleted.