“Crithidia Factor”— A New Member of the Folic Acid Group of Vitamins*

SUMMARY. A new growth factor which is more active than folic acid for the trypanosomid parasite of mosquitoes, Crithidia fasciculata, was partially purified. This factor is synthesized in relatively high concentration by Ochromonas and is an impurity in some commercial preparations of purine ribosides. Crithidia factor is tentatively identified as a polysubstituted pteridine which may be the pteridine actually involved in the biosynthesis of functional folic acid.