The distribution of fibronectin in 42 renal biopsies was investigated using an indirect immunoperoxidase technique on paraffin sections of formalin-fixed material. Biopsies were obtained from normal human kidneys and from patients with a variety of glomerular disorders. In normal glomeruli, fibronectin was present in Bowman's capsule, in the mesangium, and outlining peripheral capillary loops. A significant increase in fibronectin was observed in most types of glomerulopathy with a distribution closely related to the histopathological diagnosis. Fibronectin was diminished or absent in segmental scars, old diabetic nodules, and hyalinised glomeruli.