Background/Aims: The transglutaminases (TG2 and factor XIIIa) may contribute to the stability of arteries by cross-linking a variety of substrates, including extracellular matrix proteins and protease inhibitors. The preferred substrates have never been determined, however. Methods: We used an amine donor, 5-biotinamidopentylamine, that is covalently linked to acceptor glutamine residues, to determine transglutaminase substrates in carotid endarterectomy tissue. Results: The incorporation of 5-biotinamidopentylamine was calcium dependent, resulting in the labeling of several proteins that were detected by streptavidin-peroxidase and purified over a monomeric avidin affinity column. A major band of 42 kDa that was eluted from the column was sequenced along with 2 additional bands (80 and 65 kDa), revealing an internal fragment of vimentin, transferrin and albumin, respectively. Vimentin dimers were detected in 5 out of 5 carotid plaque homogenates. Conclusions: Vimentin is a major arterial substrate for transglutaminases. It has previously been shown that TG2 activity and vimentin contribute to vasomotor activity of arteries. Furthermore, transglutaminases (both TG2 and factor XIIIa), as well as vimentin, regulate structural alterations (inward remodeling) that occur in response to low flow states. Transglutaminase-mediated vimentin dimerization produces a novel unifying pathway by which vasodilatory and remodeling responses may be regulated.