Rabbit antibodies to human fibronectin which recognize tissue fibronectin and lack species-specificity have been employed to localize this molecule in the arterial wall of different animal species by indirect immunofluorescence. Fibronectin has been found to be consistently associated with both the inner and external aspects of the internal elastic membrane of large size arteries in mammals. Only scanty staining for fibronectin has been detected in collagen rich areas of the vessel wall, i.e. adventitia. This topographic association of fibronectin and internal elastic membrane is maintained in large size arteries (aorta, truncus arteriosus) of all species studied, including reptiles, amphibia and fish. The present findings suggest that fibronectin may mediate the contraction of vessel wall structures (i.e. elastic membranes) which lack an intrinsic contractile activity.