Phagocytosis and intracellular killing of invading pathogens by host cells play the major role in resistance to bacterial infections. In vitro, antibiotics improve the susceptibility of microorganisms to antimicrobial activity of leukocytes, suggesting that this effect may contribute to determine the antimicrobial therapy and safe dosing intervals. The susceptibility of Staphylococcus aureus to phagocytosis and killing by human polymorphonuclear leukocytes (PMNL) in the presence of normal human serum in the postantibiotic phase of fosfomycin were compared with ciprofloxacin, cefotaxime and pristinamycin. Pretreatment of S. aureus for 10 min with 4 x MIC of fosfomycin and ciprofloxacin clearly sensitized the bacteria to leukocytic killing in the presence of normal human serum (10% v/v); cefotaxime and pristnamycin failed to enhance the phagocytic killing.