Sural nerve specimens from ten patients with amyloidosis (hereditary, associated with lymphoproliferative disorders, or of unknown origin) and peripheral neuropathy were investigated by immunohistochemistry at the light and electron microscopic level. Peroxidase-antiperoxidase and immunogold techniques were applied to glutaraldehyde-fixed, osmicated and epoxy-embedded tissue. In five cases, four of which associated with lymphoproliferative disorders, amyloid deposits strongly and exclusively reacted with antibodies to kappa or lambda light chains, respectively. By electron microscopy, bundles of immunogold-labelled amyloid fibrils could be identified in coated and uncoated single membrane-bound vesicles of endoneurial macrophages. Schwann cells did not contain intracellular amyloid but their processes were entangled in amyloid fibrils and their basement membranes were sometimes fused with the fibrillar masses. It is concluded that immunoglobulin light chains in AL (amyloid of immunoglobulin light chain origin) amyloidosis precipitate, forming amyloid fibrils, in the presence of, and presumably with the assistance of, endoneurial cells. Inefficiency of phagocytosis appears to be one of the major causes for the deleterious effects of amyloid.