A cDNA clone for mouse leukotriene A4 hydrolase encoding the full sequence of the enzyme was isolated from a mouse spleen lambda ZAP-II library. The identification was ascertained by expression of enzyme activity in Escherichia coli. The encoded protein has 610 amino acids and exhibits an extensive identity (93%) with the human leukotriene A4 hydrolase. A region spanning between residues 233 and 340, where the zinc binding site is located, was found to be perfectly conserved between the two species. We found six sites of polymorphism in the cDNA sequence of mouse LTA4 hydrolase, one of which leads to a difference in the encoded amino acid. The polymorphism of cDNA was confirmed by reverse transcription-PCR sequencing of mouse spleen total RNA, prepared as a mixture from ten different strains.