Human alveolar macrophages (AMs) were obtained by bronchoscopy from 11 healthy adult subjects and placed into tissue culture for 24 hr. Brief preexposure (15 min) of human neutrophils to AM culture supernatants led to a greater than twofold increase in neutrophil killing of a serum-resistant strain of Pseudomonas aeruginosa (P less than 0.02). No increase in phagocytosis of 35S-labeled Pseudomonas could be detected for neutrophils preexposed to AM supernatants. However, upon exposure to bacteria, neutrophils preincubated with AM supernatants generated significantly more (P less than 0.05) superoxide anion than controls. This suggested that AM supernatants enhanced neutrophil oxidative bactericidal capacity. The material in AM supernatants which enhanced neutrophilic killing of Pseudomonas was less than 10,000 daltons in mass and heat-stable (56 C for 30 min). Release of this material was partially inhibitable by exposure of AMs to cycloheximide in tissue cultures. These data suggest that an AM-mediated amplification of neutrophil bactericidal capacity might be important in the defense of the human lung.