Cryostat sections of human peripheral nerves adsorbed sheep erythrocytes (E) sensitized with rabbit IgG antibodies (A) (EA). No adsorption occurred using unsensitized E or E sensitized with IgM or F(ab')2 fragments of IgG. The binding of EA was inhibited by IgG and by Fc fragments of IgG, but not by IgA, IgM, F(ab')2 fragments of IgG or albumin, and thus indicated the presence of receptors for the Fc part of IgG (FcR). IgG1 and IgG3 subclasses inhibited the haemadsorption. IgG4 was less inhibitory than IgG1 and IgG3, whereas IgG2 did not inhibit the binding of EA. The receptors were sensitive to periodic acid, formaldehyde and heat. FcR were present in sections of both unmyelinated and myelinated nerves, indicating that the binding was not to myelin. FcR were also demonstrated in sections of fetal peripheral nerves at a gestational age of approximately 21 weeks. Using immune complexes of horseradish peroxidase (HRP) and rabbit IgG antibodies to HRP and a monoclonal antibody against FcR, the receptors were shown on the nerve fibres, probably on the Schwann cell membrane (neurilemma).