Sural nerve biopsy specimens from seven patients with chronic relapsing polyneuropathy (CRP) were studied by direct immunofluorescence. Granular deposits containing IgM (7/7) and C3 (6/7) (and occasionally IgG, 3/7) were found in intraneural blood vessels. Linear deposits of IgM (6/7) (and occasionally IgG, 3/7) without C3 (0/7) were found on the Schwann cell plasmalemma (and sometimes extending deeper into the Schwann cell) of yet undemyelinated portions of nerve fibers. Albumin and fibrinogen were not found in any locus. In sural nerves of ten disease-control patients with non-dysimmune chronic perpheral neuropathies, no deposits were seen on the vessels or the nerve fibers. The Schwann cell deposits may reflect a complement-independent IgM antibody toxic to Schwann cells that underlies the pathogenesis of CRP, perhaps facilitated in its passage across the blood-nerve barrier by damage from the complement-binding IgM complexes in the intraneural vessels.