Two pteridine derivatives (Compound I and II) have been isolated from the synthetic culture fluid on which Serratia indica was grown. Compound I isolated as pale yellow spiny crystals has a strong growth-promoting activity for Crithidia fasciculata and was identified as 2-amino-4-hydroxy-6-(l-threo-1',2',3'-trihydroxypropyl) pteridine (i.e., l-thero-neopterin) by elemental analysis, by its ultraviolet and infrared absorption spectra, by its fluorescence and excitation spectra, specific rotation, by oxidative degradation study with permanganate and by paper-chromatographic comparison with synthetic l-threo-neopterin. Compound II was identified as 2-amino-4,7-dihydroxypteridine (i.e., isoxanthopterin) by comparison of its ultraviolet absorption spectra, fluorescence and excitation spectra and by paper-chromatographic behaviours with those of the authentic specimen. The biosynthesis of these pteridines by cell-free extracts of S. indica was also discussed.