To help understand effector mechanisms in experimental allergic encephalomyelitis (EAE) we examined the effects of adding 'irrelevant' lymphocytes from non-EAE donors to major basic protein (MBP)-reactive lymphocytes in the adoptive transfer of EAE (Tr-EAE). The intravenous injection of tetanus toxoid-reactive lymphocytes (TT-cells) and phytohemagglutinin-stimulated lymphocytes on the same day or 2 days after intra-arterial injection of MBP-reactive lymphocytes enhanced the clinical and pathological expression of Tr-EAE. In lymphocyte trafficking studies there was significant accumulation of these injected TT-cells in the central nervous system during enhanced transfer of EAE. W3/25-positive cells were much more predominant in lesion of central nervous system when reinjected with TT cells along with MBP cells compared with lesions of rats injected with MBP cells alone. These findings suggest possible participation of lymphocytes other than MBP-reactive cells in the expression of EAE and provide a useful model to further explore effector mechanisms in the future.