Distribution of fibronectin (FN), a major glycoproteic component of extracellular matrix was studied by an indirect immuno-fluorescence technique using a rabbit anti-FN serum on frozen sections of fresh human tissues. In the extracellular matrix, FN is localized in epithelial and glandular basement membranes (bronchial, digestive, urothelial, prostatic, thyroid and mammary glands). FN is also visualized in vascular basement membranes and pulmonary alveolar basal laminae. In lymphoid tissues. In the liver and adrenal cortex, FN has the same distribution as reticulin fibers. In the kidney, glomerular mesangium in labeled. There are also pericellular labelings on fibroblastic and muscular cell surfaces. Epithelial cells of the mammary gland show a pericellular positive staining. FN is principally involved in cell orientation and in cell extra-cellular matrix interactions.