Proteolytic fragments of fibronectin, obtained by digestion with cathepsin G, were transferred electrophoretically from sodium dodecyl sulphate (NaDodSO4) polyacrylamide gels to nitrocellulose sheets and used as antigens for monoclonal antibodies. All 9 monoclonal antibodies tested reacted with undenatured intact fibronectin or its fragments applied directly to nitrocellulose sheets. Two of the clones did not react with the NaDodSO4-treated transferred material suggesting reactivity with conformational determinants. Distinct fragments of fibronectin could be detected by several of the antibodies. None of the monoclonal or the polyclonal antibodies used reacted with the Mr = 40,000 or Mr = 30,000 gelatin-binding fragments of fibronectin. However, one of the monoclonal antibodies reacted specifically with their precursor Mr = 64,000 fragment, but apparently with its gelatin-nonbinding segment. The apparent non-immunogenicity of the gelatin-binding domain is conspicuous, suggesting that it may be highly conserved in evolution. The present method, combination of controlled proteolytic cleavage with electrophoretic transfer, provides an effective means for characterization of monoclonal antibodies raised against proteins.