An unambiguous synthesis of biopterin [2-amino-6-(L-erythro-1,2-dihydroxypropyl)pteridin-4-(3H)-one] is described; based on an improved synthesis of L-neopterin. The synthesis yields biopterin, a naturally occurring pteridine, in a pure, crystalline form without recourse to chromatography. D-Biopterin and D-neopterin are also described.