In order to further clarify the physiologic role of the neutrophil-directed chemotactic factor derived from alveolar macrophages, we evaluated those stimuli that possess the potential to regulate the quantity and kinetics of its release in vitro. In short-term culture, particulate stimuli (Staphylococcus albus, Micropolyspora faeni, zymosan, and Sepharose 4B) as well as IgG-immune complexes induced normal guinea pig alveolar macrophages to release significant quantities of this chemotactic factor. In addition, serum opsonization of particulate stimuli resulted in significant augmentation of release of the chemotactic factor from alveolar macrophages responding to these particles. This serum augmentation was associated with the fixation of C3b to the particle surface via the alternative complement pathway. Purified C3b, by itself, was also capable of inducing release of this macrophage-derived mediator. Partial characterization of this chemotactic factor revealed that it was a material of low molecular weight (400 to 600 daltons), and that it was antigenically and physically distinct from C5a. These studies suggested that the induction of chemotactic factor release from alveolar macrophages responding to microorganisms, noninfectious particulates, antigen-complexed IgG, and C3b may contribute to the pathophysiologic events observed in those lung diseases characterized by an influx of neutrophils into the pulmonary parenchyma.