The effects of teicoplanin on human macrophage functions were evaluated by assays of antibiotic uptake, bacterial phagocytosis and intracellular killing. The results indicated that teicoplanin was efficiently concentrated by both resident and stimulated phagocytes, achieving intracellular concentrations higher than those in the surrounding extracellular medium. Comparison of the degree of antibiotic penetration into dead, resident and stimulated macrophages seemed to suggest that transfer across the macrophage membrane was of a passive nature, and was not related to the metabolic state of the cells. At concentrations of half its MIC for the bacteria, teicoplanin caused macrophages to ingest and kill Staphylococcus aureus at a greater rate than did macrophages without drug. Phagocytes harvested from mice receiving intravenous teicoplanin showed greater phagocytic activity than those from control mice, suggesting that potentiation of host defences can occur in vivo.