Synthesis and breakdown of pyridoxal phosphate by erythrocytes were studied in subjects with oral lesions before and after treatment with riboflavin. In vivo conversion of pyridoxine to pyridoxal phosphate as well as in vitro synthesis of pyridoxal phosphate by erythrocytes were lower in subjects with lesions of the mouth and improved markedly after treatment with riboflavin. Erythrocyte phosphatase activity, with pyridoxal phosphate as substrate, was lower in riboflavin-deficient subjects and showed an increase after treatment with riboflavin.