Staphylococcus aureus produces synergohymenotropic bi-component leukocytolytic toxins, Panton-Valentine Leukocidin (PVL) composed of non-associated soluble proteins, LukS-PV and LukF-PV. PVL has been shown to induce the lysis of host defense cells such as human and rabbit polymorphonuclear neutrophils (PMNs), monocytes, and macrophages. A number of epidemiologic and experimental studies showed that the PVL-carrying S. aureus causes severe infection such as fulminant septicemia and necrotizing pneumonitis. Other researchers reported that the increased production of newly identified cytolitic peptides, phenol-soluble modulins (PSMs), may be responsible for the high virulence of certain community-acquired methicillin-resistant S. aureus (CA-MRSA) strains. To clarify the mechanism of the high virulence expressed by the CA-MRSA strains, we constructed recombinant PVL proteins using GST-fusion system, and evaluated their cytolytic effect against human PMNs by flow cytometric assay in the presence or absence of phenol-soluble modulin alpha 3 (PSMa3). The data showed that the PVL-mediated cytolytic activity was significantly augmented in the presence of PSMa3. Thus, simultaneous production of PVL and PSM was considered as the essence of the high virulence of PVL-producing CA-MRSA strains. We also succeeded in the production of anti-Luk-PV monoclonal antibodies that neutralize the activity of PVL with high specificities. In conclusion, our data confirmed the crucial role of PVL production in the manifestation of high virulence of the CA-MRSA strains.