Tissue distribution of human fibronectin injected intravenously into mice was studied by using immunofluorescence with species-specific antisera to human and mouse fibronectins. Human fibronectin was detected in the tissues of mice injected with human fibronectin. The distribution of the injected fibronectin was indistinguishable from that of mouse fibronectin. The staining for human fibronectin in the livers of injected mice was unaffected by perfusion of the livers prior to the preparation of tissue sections, and human IgG injected in mice as a control was not detectable in tissues by immunofluorescence. Mice were also injected with fragments of fibronectin with molecular weights close to 200,000. These fragments lack parts of the NH2 and COOH termini of the fibronectin polypeptide, including the part with the interchain disulfide bond(s), but retain the ability to bind to collagen and heparin and the ability to mediate cell attachment. They showed essentially no tissue incorporation, suggesting that the integrity of the fibronectin molecule is important for the accumulation of fibronectin in tissues. The incorporation of injected fibronectin into tissues demonstrated here suggests that circulating fibronectin contributes to the extracellular matrix of tissues.