This study was designed to identify which cells express major histocompatibility complex class II (Ia) antigen in experimental autoimmune neuritis and may therefore be antigen presenters. Serial 1-μm-thick cryosections of ventral roots of animals with experimental autoimmune neuritis were labeled with Ox6 antibody against rat Ia, the ED1 antibody to identify monocytes/macrophages and an antiserum against S100, a marker for Schwann cells. Ia-positive cells were predominantly present before overt clinical signs and demyelination (day 12). At later stages when many axons were demyelinated, their number was markedly reduced. Few Ia-positive cells that had extending long processes, which over some distance were in immediate contact with several myelin sheaths, were scattered in normal-appearing nerve roots at these later time points. Most of the Ia-positive cells could be identified as ED1-positive leanmonocytes/macrophages, but in contrast most phagocytic macrophages in advanced stages of myelin degradation no longer expressed Ia. Ia-positive structures were invariably negative for S100 at early and late stages of experimental autoimmune neurities, indicating that Schwann cells did not express identifiable Ia antigen. These findings contrast with reports of expression of major histocompatibility complex class II antigens by Schwann cells in human neuropathies. Furthermore they do not support the notion that aberrant Ia expression by Schwann cells plays a major pathogenic role in experimental autoimmune disease of the peripheral nervous system.