Cell-free extracts of Brassica pekinensis Rupr (Hakusai) catalyzed the formation of dihydropteroic acid from guanosine compounds and p-aminobenzoic acid. This reaction required ATP, Mg2+ and phosphate. GTP and GDP were more active in dihydropteroic acid synthesis than GMP and guanosine. When GTP was used as substrate, the synthesis was absolutely dependent on ATP and Mg2+. 2-Amino-4-hydroxy-6-(D-erythro-1[image] ,2[image] ,3[image]-trihydroxypropyl) dihydropteridine was the most active as a pteridine donor for the formation of dihydropteroic acid. This enzyme reaction needed ATP and Mg2+. 2-Amino-4-hydroxy-6-(D-erythro-1[image] ,2[image] ,3[image] -tri-hydroxypropyl) dihydropteridine converted to 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine without ATP and Mg2+. 2-Amino-4-hydroxydihydropteridine and 2,4,5-triamino-6-hydroxypyrimidlne were not involved in the synthesis of dihydropteroic acid. 2-Amino-4-hydroxy-6-hydroxymethyldihydropteridine pyrophosphate could be converted into dihydropteroic acid only in the presence of Mg2+. The monophosphate ester was not utilized in the conversion.