In clinical trials, copolymer I (Cop 1) appears to reduce the number of exacerbations in early relapsing-remitting multiple sclerosis. The mechanism of this effect is uncertain, but Cop 1 also reduces the severity of experimental allergic encephalomyelitis and inhibits the response of murine myelin basic protein (MBP)-specific T cells. We tested MBP-specific T-cell lines and clones from four subjects to determine whether Cop 1 also limits the human response to MBP. We found no inhibition by Cop 1 of the human T-cell response to MBP.