The yeast, Eremothecium ashbyii, produces, in addition to large quantities of riboflavin, a ribityl pteridine, Compound A, which has a structure formally related to that of the vitamin. Compound A of high specific activity was obtained from cultures of this yeast fed guanine-U-C¹⁴. One mole of Compound A consumes 3 moles of NaIO₄ and yields 1 mole of HCHO, 2 moles of HCOOH and a product having ultraviolet absorption spectra similar to those of Compound A. This last product upon reduction with NaBH₄ yields a substance identical with 6-methyl-8-(β-hydroxyethyl)-2,4,7-trioxohexahydropteridine. Comparison of 6-methyl-8-(1-D-ribityl)-2,4,7-trioxo-hexahydropteridine with Compound A in regard to their physical properties and the dilution of radioactive Compound A with the synthetic compound have established the identity of the two substances.