1. Two complementary fractions of tyrocidine-synthesizing system, Components I and II, were purified from a crude extract of Bacillus brevis ATCC 8185 by means of ammonium sulfate fractionation, protamine sulfate precipitation, DEAE-cellulose and hydroxylapatite column chromatography. Molecular weight of 100, 000 and 290, 000, were obtained for Components I and II, respectively, by sucrose density gradient centrifugation. 2. D-and L-phenylalanine activating activity was found in Component I. Component II possesseds the activiting activites of all the constituent amino acids of tyrocidine. ATP-dependent phenylalanine recemase activity was detected in Component I. Component I bound D-phenylalanine and Component II bound all constituent amino acids tested. The binding required ATP, Mg2+ ion and probably free sulfhydryl groups in the enzyme. 3. In the synthesis of tyrocidine, Component I could be replaced by the phenylalanine racemase preparation purified from Bacillus brevis Nagano, a gramicidin S-producing bacterium. 4. The results obtained clarify the function of the two components, i.e., Component I is concerned with the formation of the D-phenylalaine residue and Component II with the activation of the other amino acids and peptide formation. Tyrocidine synthesis possibly takes place through intermediates of aminoacyl-adenylate and aminoacyl-enzyme complexes.