The effect of enoxacin upon the interaction in vitro of human polymorphonuclear leucocytes with Streptococcus pneumoniae and Pseudomonas aeruginosa was examined. Synergistic killing of these two species of bacteria was observed in the presence of neutrophils and concentrations of enoxacin above the MIC. Enoxacin was able to kill intracellular Str. pneumoniae and Staphylococcus aureus. The activity of glucose-glucose oxidase, an oxygen-dependent bactericidal system which mimics part of the bactericidal system of human neutrophils, was enhanced by pretreatment with enoxacin and several other antibiotics. The effect of antibiotic pretreatment upon the bactericidal activity of an acid extract of neutrophil granules was variable.