Evidence is presented that fibronectin (FN) polypeptide chain contains a latent proteinase. Human plasma FN was cleaved with cathepsin D into three main fragments: 140-kDa and 70-kDa single-chain and 140-kDa double-chain polypeptides. Their separation was achieved according to their affinity for heparin-Sepharose. A single-chain 140-kDa fragment (H-1) was eluted in the first peak. This peptide corresponds to the already described fragment that originates from the central part of FN; it contains a low-affinity heparin-binding site, one free SH group, and a cell-binding site. After reduction and further purification by preparative polyacrylamide gel electrophoresis, this fragment revealed a spontaneous decomposition, which could be attributed to proteolytic degradation. The subfragments, ranging from 25 to 95 kDa, yielded the same proteolytically active doublet of 28-30 kDa when tested by NaDodSO4/polyacrylamide gel electrophoresis in a gel containing copolymerized gelatin or fibrinogen. The proteolytic activity was inhibited by specific SH proteinase inhibitors. The proteinase forms a labeled complex after its incubation with 125I-labeled cystatin. Neither FN, cathepsin D, nor any products from previous purification steps were proteolytically active under the conditions of the assay. It was suggested that the same fragment may also yield an inhibitor, since structural analogies were found between the cell-binding region of FN and SH proteinase inhibitors.