Recent evidence suggests that the selection of an antibiotic for the treatment of infection should take into account not only the drug's antimicrobial activity but also the way in which it interacts with host defence mechanisms, since this may influence the response to infection. In this study we have investigated the effects of imipenem on the activities of human macrophages against Staphylococcus aureus. Bacterial susceptibility to phagocytosis and intracellular killing were determined after S. aureus and macrophages were incubated both simultaneously with sub-inhibitory and bactericidal concentrations of imipenem and following pre-exposure of the organisms and the macrophages individually to the same concentrations of imipenem. At both concentrations and under both circumstances, imipenem potentiated the phagocytic and microbicidal activities of the macrophages.