Each of the two type II domains and four larger fragments, containing one or two type II domains of fibronectin, have been expressed in Escherichia coli. A special vector, containing a fragment encoding the cleavage site for Factor Xa, Ile-Glu-Gly-Arg, inserted immediately before the protein fragment of interest, was used. After treatment of the purified fusion proteins with reduced/oxidized glutathione, the correctly folded fibronectin fragments were released by proteolytic digestion with Factor Xa. The largest fragment, consisting of two type II and two type I domains, was the only fragment able to bind to immobilized gelatin.