Primary lung fibroblasts were isolated from patients with idiopathic pulmonary fibrosis (HIPF), from normal human lung tissue (NH), from rats treated with 75% oxygen and paraquat (PA), and from normal adult rats (NR). Serum-free media conditioned by each fibroblast strain were tested on the human A549 cell line (HIPF and NH media) or on primary alveolar epithelial cells (AEC) isolated from normal adult rats (PA or NR media). Over 20-h incubation, HIPF- or PA-conditioned media induced DNA fragmentation and significant decreases in total recoverable DNA and cell number of A549 or AEC, respectively; NH or NR media had no significant effect relative to serum-free unconditioned media. Apoptosis of A549 and AEC was detected by altered nuclear morphology and was confirmed by terminal deoxynucleotidyl transferase-mediated bio-dUTP nick end labeling. The endonuclease inhibitors 10 microM aurintricarboxylic acid and 50 microM zinc inhibited HIPF-induced apoptosis of A549 cells by 68 and 71%, respectively. Both apoptosis and necrosis were induced by HIPF and PA media in a concentration-dependent manner. These results suggest that altered fibroblasts emerging during fibrotic lung injury release a soluble factor(s) capable of inducing cell death and net loss of AEC.