Differences in domain structure between human fibronectins obtained from pericellular matrix and plasma have been revealed by limited proteolysis and S-cyanylation, followed by identification of each domain with domain-specific antibodies. Although the overall domain structure is similar between pericellular and plasma fibronectins, the fragments derived from the COOH-terminal region of these fibronectins, which were defined by specific antibodies, exhibited clear differences in their molecular weights and protease susceptibility, suggesting that the structure near the COOH-terminal region is significantly different between these two proteins.