Fibonectin is a major constituent of the fibroblast extracellular matrix. Fibronectin binds to collagen, mediates fibroblast adhesion to collagen, and is synthesized and secreted into the medium of cultured fibroblasts. Affinity-purified antibodies to fibronectin and collagen were localized using the peroxidase-antiperoxidase method or with ferritin-couple secondary antibodies. Using human fibroblasts cultured under routine conditions, fibronectin and procollagen I react in a nonperiodic manner with: 1) ~10 nm extracellular fibrils, 2) cell membrane, and 3) membrane-associated vesicles. All fibrils react with both antibodies, suggesting some form of codistribution of fibronectin and collagen in these fibrils. Treatment with ascorbate leads to the development of a larger diameter extracellular fibril, approximately 40 nm in diameter. These large diameter fibrils are clearly collagen fibrils as documented by the procollagen antibody reaction. Importantly, fibronectin is bound to or a constituent of these 'native' or cellular made collagen fibrils. Fibronectin and procollagen antibodies localized with the peroxidase-antiperoxidase method have a 70 nm axial repeat of reaction product on ascorbate-treated fibroblasts. Localization of antibodies with ferritin-labeled secondary antibodies is less satisfactory, but supports the basic observations made with the unlabeled antibody enzyme method. This observation rules out any potential criticisms. Although it is more difficult to observe with immunoferritin, there is an indication that antibodies to fibronectin react with an axial periodicity on cellular produced collasgen fibrils.