Immunoelectron microscopy using antibodies recognising Class II MHC antigens and a pan T cell marker was employed to study sections of spinal cord from guinea pigs with chronic relapsing experimental allergic encephalomyelitis (CREAE). It was found that endothelial cells expressed Class II antigens on their luminal surface throughout the course of the disease and that lymphocytes were adherent to these surfaces. In the parenchyma lymphocytes, macrophages and possibly microglia expressed Class II antigens suggesting that they might also be involved in antigen presentation. The different distribution of T cells seen in the individual lesions during the relapse phase may correlate with their respective natural histories.