Abstract Many toxic chemicals have been shown to produce an increase in lung collagen. Biochemical measurements of total lung hydroxyproline are a convenient method to quantitate and to follow disease processes associated with such an increase. However, biochemical measurements alone do not distinguish various pathological lesions and histology is needed to provide additional important information. Cell kinetic studies enhance histopathology in showing the dynamics of cell proliferation and tissue renewal. When these methods of studying lung collagen are applied to different toxic agents, such as butylated hydroxytoluene, methylcyclopentadienyl manganese tricarbonyl, CdCl 2, or anticancer drugs, different patterns of hydroxyproline accumulation and cellular kinetics are revealed. It is speculated that the development of fibrotic lung disease may be at least partially determined by the initial lesion and that quantitative analyses of cell kinetic pattens help to understand and to predict the nature and evolution of disease processes.