The liposidomycins are a family of nucleoside antibiotics, recently isolated from Streptomyces griseosporeus,' which strongly inhibit bacterial peptidoglycan synthesis. Liposidomycins inhibit formation of the lipid intermediate in peptidoglycan synthesis (unpublished data), as does tunicamyciq2 but with three orders of magnitude greater activity (ID,o 0.03 bg/mL) and extremely high specificity. For liposidomycin B, one of the principal constituents, we propose structure 1, a novel lipid-containing nucleoside of unusual complexity. Compound l resembles the reaction intermediate between UDP-N-acetylmuramylpentapeptide and undecaprenyl phosphate in the lipid cycle of peptidoglycan synthe~is.~