The bactericidal action of norfloxacin (NFLX) was studied using Staphylococcus aureus FDA209P. NFLX induced lysis of S. aureus, and the lysis was inhibited by the addition of lipoteichoic acid (LTA) or cardiolipin, which are believed to be inhibitors of autolytic enzymes. The bactericidal effect of NFLX was also diminished by the addition of LTA. These results suggest that NFLX-induced lysis is involved in the bactericidal action of this antibiotic against S. aureus.