Antimicrobial agents may interact with polymorphonuclear leukocytes and directly modify the leukocyte functions, or bacteria can be modified by the antimicrobial agents causing them to be treated differently by phagocytic cells. Brief exposure of bacteria to high levels of antimicrobials can affect their interaction with leukocytes. The susceptibility of Escherichia coli to phagocytosis and killing by human polymorphonuclear leukocytes in the presence of normal human serum following treatment with RP7293, a new natural streptogramin, was examined, and a comparison made with clavulanic acid/amoxicillin, ciprofloxacin and fosfomycin. Pretreatment of E. coli for 10 min with 4 x MIC of RP7293 clearly sensitized the bacteria to leukocytic killing in the presence of normal human serum (10% v/v).