Alveolar macrophages (AM) have many Fc receptors for IgG, but are less reactive to lymphokines. They have a well-developed oxidative metabolism and contain large amounts of lysosomal enzyme. This suggests that the antibacterial antibody plays an important role in early resistance by AM to intracellular bacterial infection and that a bactericidal agent, dependent on oxygen and lysosomal enzyme, participates in the effects of the antibacterial antibody on bactericidal activities of superoxide (O2-) and lysosomal enzyme from rabbit AM. The number of Listeria monocytogenes in AM increased after pretreatment with saline or normal IgG but decreased by 60% after pretreatment with anti-Listeria and 120 min incubation. Alveolar macrophage-phagocytized Listeria monocytogenes and Bacille Calmette Guérin (BCG) bound with antibacterial antibody enhanced release of O2-, and nitroblue tetrazolium (NBT) formazan reduced by O2- was observed around the bacteria in the phagosomes of AM. We also confirmed that Listeria and BCG were killed extracellularly by O2-released by a superoxide-generating system in vitro and/or by lysosomal concluded that the antibacterial antibody of the IgG class enhances the antibacterial activity of AM thereby increasing the production of 02- and lysosomal enzyme in the phagosome. This finding may be important in the early resistance to intracellular bacteria infection by AM in the alveolar spaces.