During the complaisant phase of murine pertussis, Bordetella pertussis survived within lung macrophages. Bactericidal activity in the complaisant mouse, which was shown to be activated by viable superinfection, could also be elicited by alcohol killed B. pertussis. Isolated complaisant cells in vitro, although not superior to normal cells in phagocytosis, showed an enhanced antibacterial activity against large numbers of viable B. pertussis. The relationship of cytophilic antibody to this enhanced bactericidal activity is unknown, but circulating protective antibody was relatively unimportant at this stage of infection. Although a higher titre of opsonins was found in complaisant mice, opsonization with optimal quantities of normal or complaisant phase serum neither increased phagocytosis nor affected the outcome of intracellular digestion.