The xanthine base fraction was prepared from 200 1. human urine according to the method of Koschara (Hoppe-Seyler''s Zeitschr. Physiol. Chem., 240: 144, 1936). To obtain the yellow pigment, xanthopterin, a 30 cm. Buchner funnel was packed with 120 g. Frankonit KL, and washed with 1 1. phosphate buffer, pH 7.6. Two 1. of the xanthine base soln., adjusted to pH 7.6 with trisodium phosphate, were passed through the filter, and the material eluted with borate buffer, pH 9.2. This soln. was subjected to fractional Ag salt precipitation, the Ag precipitate decomposed with dilute HC1, and the xanthopterin, (C6H5O2N5)x, precipitated with Na acetate. Xanthopterin was also synthesized by mixing 23.8 g. of 2.4,5-triamino-6-hydroxypyrimiden with 80 g. glyoxylic acid-Ba bisulfite and 300 cc. 78% H2SO4 for 1 hr. at 95[degree]. The mixture was cooled with ice, 100 cc. H2O added, and centrifuged. Xanthopterin sulfate crystallized from the dark red, supernatant soln.