Because of their coccidiostat effect in poultry and other biological activities of interest for agricultural use, new polyether antibiotics have appeared with increasing frequency in recent years. WESTLEY has suggested a classification into four groups by structure: monovalent polyether, monovalent monoglycoside polyether, divalent polyether and divalent pyrrole ether. However, prior to a complete structure elucidation, a more detailed classification of a potentially novel antibiotic is desirable for comparative purposes with ionophores reported in the literature. For this purpose, 13C NMR spectroscopy is extremely useful because it provides unequivocal information about all the carbon atoms, especially ketal and acetal carbon atoms generally found in this family of antibiotics. Moreover, the 13C signal assignments of known antibiotics, even if incomplete, can be used to assess the structure of a similar unknown polyether, and the results can be used for further assignments of uncertain 13C signals. As an example, we report here that antibiotic K-41B belongs to a new class of polyether type antibiotics which should be added to WESTLEY'S classification. Antibiotic K-41B was isolated as a minor component from the mother liquor of K-41A (formerly called K-41), which had been produced by a strain of Streptomyces hygroscopicus. The antibiotic was purified as a sodium salt (1-Na), m.p. 185-186°C (decomp.), [α] +4.3°C (MeOH), and is active against Gram-positive bacteria like K-41A. By comparing the 13C signals of 1-Na with those of K-41A-Na (2-Na, whose structure has been established by an X-ray analysis and signal assignments referencing known antibiotics and derivatives), it was found that 1-Na essentially corresponds to 2-Na but has seven additional signals assignable to the second deoxysugar (Deo) moiety. Since 1-Na lacks the 15-OMe signal, O-Deo is likely to be situated at C-15 instead of the OMe group, supported by slightly shifted signals of C-13, C-16, 16-Me, and C-15. Thus, the structure of K-41B-Na was concluded to be 1-Na, with elemental analysis agreeing well with C54H81O20Na. K-41B is the first example of a monovalent diglycoside polyether and the first known polyether antibiotic having eight rings in a molecule, with the largest molecular weight (1061.28) in this family.