Injury to the capillary endothelium and to alveolar epithelial cells of the lung may result in damage to the underlying collagen of the extracellular matrix. To examine this possibility, whole body irradiation, bleomycin injections, and exposure to hyperoxia were used to induce various types of lung damage in mice. The morphology of the lung and the cellular and protein content of bronchoalveolar lavage fluid were used to assess injury. Collagen breakdown was assessed from the hydroxyproline concentrations in bronchoalveolar lavage fluid. When lung cell injury was observed, protein leaked in to alveoli and hydroxyproline was detected in bronchoalveolar lavage fluid. An increase in hydroxyproline followed endothelial damage by irradiation and was greatly increased when type 1 epithelial cell necrosis also occurred after bleomycin injection or hyperoxia. Maximal concentrations of hydroxyproline occurred in mice showing respiratory distress after six days of hyperoxia. Concentrations returned to zero during the subsequent phases of cell regeneration and fibrosis seen after bleomycin injection and irradiation. There was little change in the cellular components of bronchoalveolar lavage fluid at any time. The results indicate that collagen breakdown occurs during acute lung injury and can be quantified in terms of the hydroxyproline concentration in lavage fluid. Such a change in the extracellular matrix might influence the subsequent division and differentiation of regenerating cells during repair.