OP-145 is a synthetic antimicrobial peptide developed from the human cathelicidin LL-37. The purpose of this investigation was to evaluate the effect of the antimicrobial peptide OP-145 against clinically isolated drug-resistant strains. Ten methicillin-resistant Staphylococcus aureus (MRSA) strains were obtained from our hospital's clinical inspection center, and the activity of OP-145 on growth and biofilm formation of these strains was evaluated by colony counts and scanning electron microscopy. The antimicrobial peptide OP-145 showed significant antibacterial activity against 9 MRSA strains. For the biofilm experiments, MRSA counts in the biofilms decreased significantly after 24 h (P ＜ 0.05). OP-145 strongly reduced growth and biofilm formation of clinically isolated drug-resistant strains in vitro, and the use of this class of antimicrobial agents may be an important new approach in controlling bacterial infections.