The novel macrocyclic polyketide, 10-hydroxymethyl-archazolid-7-O-b -D-glucopyranoside (archazolid D), was obtained from the myxobacterium Cystobacter violaceus. The structure of this first hydroxylated archazolid was determined by spectroscopic analysis, in particular by HMBC, HMQC, and ROESY NMR investigations, and by degradation. This novel metabolite was evaluated for growth inhibition of murine connective tissue cells and V-ATPase inhibition in comparison to other known archazolids. This subunit forms an oligomer, building up a ring structure of six or more copies which transports protons across the membrane. Recently, the structure of the ring from a bacterial V-ATPase has been described at 2.1 Å resolution and showed that 10 copies form the c ring [4]. This increasing molecular understanding in combination with the very promising biological profile of the archazolids renders these macrolide antibiotics attractive structures for further development. Very recently, the full stereochemistry of the archazolids has been determined by extensive NMR studies and chemical derivatization [5], and was confirmed by total synthesis [6]. So far, however, only very limited SAR-data are available, relying on modifications of the hydroxyl groups at C-7, C-15 and C-1Ј [7,8]. Herein, we describe the isolation and structure elucidation of the first archazolid with a modified carbon backbone, the hydroxylated derivative 10-hydroxymethylarchazolid-7-O-b -D-glucopyranoside (archazolid D, 4), from fermentation broths of the myxobacterium Cystobacter violaceus and report on the V-ATPase inhibitory effect of this novel macrolide antibiotic in comparison to other known V-ATPase inhibitors.In a screening for novel and more efficient archazolid producing myxobacteria, we have recently identified C. violaceus strain Cb vi105 as the natural source of the first archazolid glycoside, archazolid-7-O-b -D-glucopyranoside (archazolid C, 3, Fig. 1), which was obtained as the major metabolite from fermentation broths together with the parent compound archazolid A. A detailed analysis of this