Sepiapterin deaminase has been purified approximately 6,000-told from the larval integument of the lemon mutant of the silkworm by several column chromatographic procedures. Sepiapterin and isosepiapterin were active substrates among various pteridines tested. The molecular mass of this enzyme was estimated to be 74 kDa by SDS-PAGE and 70 kDa by gel filtration, suggesting that the native form of the enzyme is monomeric protein . All silkworm strains, to the best of our knowledge, had an activity of the enzyme and the enzyme was widely distributed in the larval tissues. Sepiapterin deaminase may have an important function on the silkworm.