A fraction of methicillin-resistant Staphylococcus aureus (MRSA) shows resistance to vancomycin (VCM) in the presence of β-lactam antibiotics (BIVR) at low concentrations. We hypothesized that the BIVR phenomenon might be exerted by a peptidoglycan derivative(s) generated as a consequence of β-lactam antibiotic action. To verify this hypothesis, we isolated the fraction that mimicked the effect of β-lactam antibiotics by the enzymatic treatment of the crude cell wall. The active components were purified by a combination of reverse phase chromatographies, mass spectrum and amino-acid analyses, and were identified to be a muropeptide with the following formula: N-acetyglucosamyl-N-acetylmuramyl--Ala-D-isoGln-L-Lys-(ɛ-NH-4Gly)-D-Ala-2Gly. This is the very first identification of the active component, which induces VCM resistance in MRSA. We found that the BIVR cells are highly sensitive to this compound rendering the cells resistant to VCM compared with non-BIVR MRSA.