In Drosophila and silkworm (Bombyx mori), the metabolic relationsphip among several pteridine compounds has been established biochemically and genetically. Furthermore, the problems concerning the structure of the yellow pigment, the precursor of pteridines and the related enzymes have been clarified. Inheritance of lem lethal in the silkworm, particularly its maternal effect, has been studied. The lethal and its maternal mechanisms have been interpreted in terms of the relation of the gene to the enzyme, pterine reductase. A specific protein combines with the yellow pigment, and is necessary for maintaining the pigment in the hypodermal cells of the silkworm. The nature of the protein has been discussed genetically and biochemically.