RNAIII-inhibiting peptide (20 mg/kg, i.v.) decreases lethality in mice challenged with S. aureus ATCC 25923, with the lethality rate of 70%, and when combined with cefazolin, imipenem, vancomycin, the rates are 20%, 15% and 10%. RNAIII-inhibiting peptide (20 mg/kg, i.v.) also causes decreased lethality in mice challenged with S. aureus Smith, with the lethality rate of 75%, and the lethality rate decreases to 30%, 10% and 10% when combined with cefazolin, imipenem, vancomycin.