Experimental allergic encephalomyelitis (EAE) can be adoptively transferred in Lewis rats with spleen cells from immunized animals, after culture with concanavalin A or myelin basic protein (MBP). The effect of the immunosuppressive drug cyclosporin A (CsA) on the in vitro and in vivo steps of the cell transfer has been investigated. Clinical signs of EAE were completely suppressed by CsA in rats immunized with MBP in Freund's complete adjuvant and spleen cells from these animals, cultured with the antigen, did not transfer the disease. The incidence of transferred disease was also reduced, if recipients were treated with CsA, although a higher dose of drug than that needed to suppress active EAE was required. In both instances complete suppression of EAE was only accomplished for the period of dosing, although the clinical signs of disease which appeared after withdrawal of the drug were significantly reduced in severity. These results imply that an immune response in the host animal is a prerequisite for adoptive transfer of EAE or suggest that CsA can regulate the action of lymphocytes already primed.