To provide novel methods of treating Staphylococcus aureus (S. aureus)-related septicemia and immunocompromised pneumonia and the like.SOLUTION: Provided is a method for preventing S. aureus-related sepsis or reducing severity thereof in a mammalian subject, comprising administering to the subject an effective amount of an isolated anti S. Aureus α toxin (anti-AT) antibody or antigen-binding fragments thereof. The anti-AT antibody is also useful to reduce the bacteria load of S. aureus in a bloodstream or heart； to reduce S. aureus aggregation and/or thromboembolic lesion formation； and to prevent S. aureus-related pneumonia or alleviate the severity thereof； and the like.SELECTED DRAWING: Figure 1【課題】黄色ブドウ球菌（Ｓ．ａｕｒｅｕｓ）関連性敗血症、免疫無防備状態の肺炎等の新規治療方法の提供。【解決手段】哺乳動物対象における黄色ブドウ球菌（Ｓ．ａｕｒｅｕｓ）関連性敗血症を予防する、又はその重症度を軽減する方法であって、有効量の単離抗黄色ブドウ球菌（Ｓ．ａｕｒｅｕｓ）α毒素（抗ＡＴ）抗体又はその抗原結合断片を前記対象に投与することを含む方法。前記抗ＡＴ抗体は、血流又は心臓における黄色ブドウ球菌（Ｓ．ａｕｒｅｕｓ）の細菌負荷の低減；黄色ブドウ球菌（Ｓ．ａｕｒｅｕｓ）凝集及び／又は血栓塞栓性病変形成の低減；黄色ブドウ球菌（Ｓ．ａｕｒｅｕｓ）関連性肺炎の予防又はその重症度の軽減等にも有用である。【選択図】図１
