We studied distribution of fibronectin in paraquat-induced adult fibrotic lung by indirect immunofluorescence and indirect immunoperoxidase methods, using affinity-purified antifibronectin IgG peroxidase conjugates and antifibronectin IgG FITC conjugates. In contrast to the relative paucity of staining in normal lung, there was a marked increase in interstitial staining for fibronectin in this fibrotic lung. This marked alterations in the apparent amounts and distribution of fibronectin in fibrotic human lung suggest its involvement in the cellular events accompanying human lung fibrosis.