• The immunohistochemical staining of immunoglobulins (Ig), complement (C3), and fibrinogen in chronic relapsing experimental allergic encephalomyelitis lesions showed different staining patterns in the acute vs the chronic stage of the disease. In the acute stage, Ig, C3, and fibrinogen were present in the perivascular tissue of the brain and the spinal cord. In hyperacute-type lesions, the binding of Ig and C3 to the parenchyma was especially pronounced. The chronic stage of the disease was characterized by lg-containing cells and Ig binding to white matter in actively demyelinating lesions. Linear or granular deposits of immunoglobulin and complement, reminiscent of those described in immune complex disease, were found in the choroid plexus.