From the structural investigation of acidic products obtained by hydrolysis of enduracidins and tetrahydroenduracidins, 10-methylundeca-2(cis)-4(trans)-dienoic acid (V) and (+)-10-methyldodeca-2(cis)-4(trans)-dienoic acid (VI) were found to be constituent of enduracidins A and B, respectively. Consequently, the structural difference between enduracidins A and B was proved to be present in their unsaturated fatty acid moieties, and their full structures were assumed to be as shown in Fig. 10. Enduracidin was obtained from the mycelium of Streptomyces fungicidicus B-54773 and found to be a novel basic peptide antibiotic containing chlorine in the molecule, showing strong bactericidal activity both in vitro and in vivo against gram-positive bacteria including strains resistant to known antibiotics. Enduracidin was separated into enduracidin A (I) and B (II) by Amberlite XAD-2 column chromatography with gradient elution of 0.05N NaCl to 0.006N HCl in 50% aqueous methanol, but both components have closely similar physicochemical properties. Hori, et al. investigated structures of the peptide moieties of I and II, finding they are composed of seventeen amino acids, with sixteen forming a macrocyclic peptide lactone. In this paper, the chemical structures of novel fatty acids obtained from I and II are discussed, the difference in the chemical structure between I and II is proved to lie in their fatty acid moieties, and the full structures of I and II are presented.