The localization of type I, III, IV, and AB2 (V) collagens in normal and fibrotic human lungs was studied by indirect immunofluorescence techniques with affinity purified antibodies specific for these types of collagen. In the normal lung type I collagen appears to be localized to the interstitium of alveolar septa in irregular patterns. Type III appears to have a more prominent, irregular localization in septa and a perivascular localization. Types IV and V codistribute in linear patterns on alveolar and capillary basement membranes. Type V is also present in the interstitium. Dramatic changes in relative amounts and localization were noted in fibrotic lung samples by fluorescence microscopy. In fibrotic lung samples there is a marked increase in type I in thickened septae. Type III is markedly reduced and seen only in perivascular localization. Type V is markedly increased in the interstitium and located in areas of smooth muscle cell proliferation. No apparent change is noted in amount or localization of type IV collagen. These findings correlate well with biochemical data relating to the amounts of various collagens isolated by enzymatic extraction techniques. These results are discussed with respect to staging and evolution of the fibrotic process.