Adult strain 13 guinea pigs in the relapsing-remitting phase of chronic relapsing experimental allergic encephalomyelitis were treated with cyclosporin-A (CS-A) at an oral dose of 35 mg kg-1 body wt. for periods of 17-32 days. Of the animals treated in relapse or remission the majority (6/11) showed a deterioration in clinical condition during treatment with the remaining 5 showing no change and 6/8 developed more severe clinical signs after treatment had ceased. In contrast, a similar group of untreated or vehicle-only treated relapsing-remitting animals developed a worse or better clinical condition in similar numbers when assessed over a similar time period with the majority (22/33) showing no change in clinical signs. Blood white cell counts and plasma IgG values remained constant throughout treatment and no clinical signs of drug toxicity were evident. Cerebrospinal fluid (CSF) mononuclear cell counts rose initially only in CS-A treated animals but fell to levels below those of controls by the end of treatment. These results are discussed in the context of the effectiveness of CS-A therapy in autoimmune diseases of the central nervous system.