These experiments were conducted to study the possible involvement of macrophage-derived gelatinases in the bleomycin-induced model of pulmonary fibrosis. Normal rat alveolar macrophages and the rat alveolar macrophage cell line NR8383 were stimulated in vitro with 0-1.0 microgram/ml bleomycin for 18 h. Gelatinase activity in the medium was assayed on zymograms in which gelatin or collagen were used as substrates. Macrophages stimulated with 0.01-1.0 microgram/ml of bleomycin secreted significantly more of a 92-kDa gelatinase than did unstimulated controls. Addition of cycloheximide during stimulation decreased gelatinase activity by 86 +/- 4%, and activity was completely inhibited by the addition of EDTA to zymograms. This gelatinase degraded denatured type I collagen and native type IV collagen. Western blot analysis using a monoclonal mouse anti-rat antibody demonstrated that this enzyme was the same as a metalloproteinase secreted by rat mammary carcinoma cells. Gelatinase secreted by macrophages in fibrotic lungs may enhance macrophage migration through the lung and may also be active in the remodeling process.