Teicoplanin, a lipoglycopeptide antibiotic, consists of five major components (A2-1 through A2-5), one hydrolysis component (A3-1), and four minor components (RS-1 through RS-4). All the major components contain an N-acyl-beta-D-glucosamine, but they differ in the lengths and branchings of their acyl-aliphatic chains. Previous studies with radiolabeled teicoplanin in rats and humans have shown that the drug is eliminated by the renal route and that metabolic transformation is very minor, about 5%. A possible metabolic transformation of teicoplanin into A3-1 was also suggested. In the present study in humans, two metabolites (metabolites 1 and 2; 2 to 3% of total teicoplanin) were isolated after intravenous administration of radiolabeled teicoplanin. After purification, their structures were determined by fast atom bombardment mass spectroscopy and 1H nuclear magnetic resonance spectroscopy on the basis of the well-known correlations established in this field, and they were found to be new teicoplaninlike molecules, bearing 8-hydroxydecanoic and 9-hydroxydecanoic acyl moieties. This metabolic transformation is likely due to hydroxylation in the omega-2 and omega-1 positions for metabolites 1 and 2, respectively, of the C-10 linear side chain of component A2-3. This might explain the low extent of metabolism of teicoplanin if we consider that only component A2-3 has a linear chain that is susceptible to such oxidation.