N ormal and antibiotic-pretreated staphylococci were incubated with human neutrophils to determine the interactions between cells and antimicrobials in the killing of the organisms. Staphylococcus aureus 502A pretreated during log-phase growth with subinhibitory (¼ minimum inhibiting concentration) (MIC) concentrations of penicillin G were more susceptible to killing by normal neutrophils than untreated bacteria (intracellular survival 0.17±0.04 vs. 1.5±0.38%, mean±SEM, respectively, at 35 min in 14 experiments; P < 0.01 by t test). Furthermore, this enhanced susceptibility to killing was observed even when phagosome formation was inhibited by cytochalasin B (65.6±4.6% pencillintreated vs. 30.5±4.5% untreated killed at 30 min in 14 experiments, P < 0.001). Pretreatment of S. aureus with vancomycin similarly enhanced susceptibility to killing by cytochalasin B-treated polymorphonuclear leukocytes (PMN), whereas pretreatment with gentamicin did not.The enchancement of killing by pretreatment with cell wall-active antibiotics was present in a dose-response fashion to 1/16th the MIC. It required specific antimicrobial activity; i.e., penicillin activity was inhibited by penicillinase or by incubation with bacteria at 4°C. It also required active cellular metabolism and intact neutrophils. For antibiotic-pretreated bacteria to be killed by normal and cytochalasin B-treated cells, phagocytosis or binding to the cells was essential via a serum opsonindependent mechanism. In experiments with the cytochalasin B-treated cells, all bound penicillin-treated bacteria were killed vs. only a fraction (70%) of the bound untreated bacteria. Penicillin in 10 times the MIC had no direct effects on PMN phagocytic, metabolic, or microbicidal functions against a nonsusceptible organism, Candida albicans.The results indicate a cooperative effect between cell wall-active antibiotics at low concentrations and human PMN in the killing of staphylococci. The model establishes conditions for the study of the mechanisms involved in the cooperation of these bactericidal systems.