The trypanosomid flagellate Crithidia fasciculata was found to have a growth requirement for certain 6-substituted pteridines. The most active compound studied was 2-amino-4-hydroxy-6 (1'', 2''-dihydroxypropyl) pteridine ("biopterin") which was isolated from human urine. A requirement for both pteroylglutamic acid and an appropriate pteridine was demonstrated for the organism.