The in vitro antistaphylococcal activity of lactoferrin and the antibiotic resistance of clinical Staphylococcus aureus isolates obtained from three different sites of infection were examined. Antibiotic, but not lactoferrin resistance correlated with selective antibiotic pressure, and nosocomial and most community isolates were antibiotic resistant, whereas only a third of each group was resistant to lactoferrin. The antimicrobial activity of lactoferrin, both in defined medium and in normal human plasma serum, was dependent upon its ferrochelating properties. Therapeutic approaches based on the use of ferrochelating agents such as lactoferrin combined with antimicrobial drugs may help to counteract the reduced efficacy of current antibiotics.