The antibody and cell mediated immune responses were investigated in inbred Lewis rats with experimental allergic neuritis (EAN) induced by either P2, a protein purified from the bovine cauda equina nerve roots, or whole bovine nerve root myelin. In the P2 immunised animals both antibodies to P2 detected by radioimmunoassay and cell-mediated immunity to P2 assayed by skin testing appeared before the onset of EAN and persisted during and after the disease. In the myelin immunised animals the antibody titres were lower and somewhat delayed and the skin tests became negative at the height of the disease. Complement-fixing antibodies to galactocerebroside, which have been implicated in the production of demyelination under some circumstances, could not be detected in the serum after immunisation with either P2 or myelin. EAN was transferred passively with lymph node cells from rats immunised with either P2 or myelin although anti-P2 antibodies could not be detected in the serum of recipients with EAN. The results favour a cell-mediated immune response to P2 as the most important pathogenetic mechanism in EAN induced wtih whole myelin in the rat.