The in vitro effects of moxalactam and gentamicin on chemotactic, phagocytic, and microbicidal activities of human polymorphonuclear leukocytes have been studied. Incubation of cells with moxalactam (25 to 400 micrograms/ml) resulted in a 10 to 15% inhibition of leukocyte chemotaxis (P less than or equal to 0.10). Incubation of cells with gentamicin (5 to 40 micrograms/ml) inhibited chemotaxis more than 50% (P less than or equal to 0.50). Neither moxalactam nor gentamicin had any effect on phagocytic or microbicidal capacities of leukocytes.