Asbestosis is generally considered to result in restrictive pulmonary disease associated with interstitial fibrosis. Recently, however, attention has focused upon bronchiolar lesions and concomitant obstruction to air flow. The responses of conducting airways and alveoli were studied over a 20 week period following instillation of crocidolite to mice. The location of the lesions and the sequential inflammatory changes were studied by bronchoalveolar lavage, light and electron microscopy; regenerative activity was monitored by autoradiographs. Within 48 h there was multifocal necrosis of bronchiolar epithelium, maximal at bifurcations where longer fibres tend to adhere. Subsequently, intralumenal exudates were overgrown by epithelium and incorporated into the bronchiolar connective tissue where active peribronchiolar granulomas persisted until 20 weeks. Alveolar lesions were located predominantly in peribronchiolar air sacs and at the junctions of bronchioles and alveolar ducts. Focal acute injury of type 1 cells and transepithelial passage facilitated transport of short asbestos fibres to the interstitium where they were phagocytozed by macrophages. Regenerative activity was prompt with active division of type 2 epithelial cells. Biochemically, collagen increased after 4 weeks, when fibrosis was predominantly located in the bronchiolar lumens and in peribronchiolar connective tissue with lesser amounts in the centrilobular alveolar interstitium. The results suggest that the longer fibres induce bronchiolar injury and a more severe fibrotic pattern similar to the recently described changes seen in human asbestosis.