Many antibiotics in addition to their direct effect on bacteria can modulate host defence in different ways. The influence of cefotaxime on the phagocytic and bactericidal activity of mouse peritoneal macrophages was investigated. At concentrations of half the MIC the antibiotic caused macrophages to kill intracellular Staphylococcus aureus at a higher level than did macrophages without drug. Bacteria pretreated with cefotaxime became more sensitive to macrophage phagocytic and bactericidal activity. Macrophages taken from mice receiving intravenous cefotaxime showed greater phagocytic capacity than those from control mice.