Complement-dependent demyelinating activity of whole brain homogenate (WBH)-induced experimental allergic encephalomyelitis (EAE) sera was tested on long term tissue cultures of in vitro myelinated fetal guinea pig cerebellum. Complement-fixing (CF) auto-antibodies were shown to be the responsible agents, as demonstrated in experiments where all reagents belonged to the same species: guinea pigs of outbred (Hartley) and even of inbred (S2 or S13) strains. These antibodies were of the IgG2 class as shown by Sephadex G-200 and DEAE cellulose fractionation experiments. The corresponding auto-antigen was present in the homogenate and myelin of the central nervous system (CNS) tissue. It was different from the encephalitogenic basic protein of CNS myelin (BP), as shown in experiments where the demyelinating auto-antibodies were induced, detected, and absorbed by WBH or by CNS myelin but not by BP. They were neither induced by nor cross-reacting with cerebroside and peripheral nervous system (PNS) tissue.