In this study, we report a strong interaction between two contiguous proteolytic fragments of fibronectin, each having a mass of about 16 kDa. This interaction was stable in 4 M NaCl and 4 M urea and dissociation of the two fragments required buffers containing 0.5% sodium dodecyl sulphate. After purification, these peptides maintained their ability to interact when mixed. One fragment was made up of type III repeat 4 and part of 5, the other by repeat 6 and part of 5. Such strong interaction between two fibronectin regions may play a role in fibronectin conformation as well as during fibronectin fibril formation.