In order to assess the potential role of local production of complement in pulmonary host defenses against bacterial infection, this aspect of bronchoalveolar macrophage function was studied in guinea pigs challenged with Pseudomonas aeruginosa in an acute and chronic infection model. Acute infection resulted in an increase in bronchoalveolar macrophage cell number and an increase in synthesis and secretion rates for the second (C2) and fourth (C4) complement components per macrophage. Manipulation of the airway without introduction of Pseudomonas also increased synthesis of both C2 and C4 when studied 60 h after control solutions were administered. Pseudomonas aeruginosa delivered in agar beads to induce chronic inflammation resulted in specific stimulation of C2 and C4 synthesis at 2 wk and to a lesser extent at 4 wk postchallenge. This increase in local complement synthesis by bronchoalveolar macrophages, in addition to enhancing the local inflammatory response, may serve to facilitate recruitment of intravascular cellular and humoral mediators of host defense against bacterial infection.