Myelin antigen–reactive T cells have been implicated in the pathogenesis of multiple sclerosis (MS). Myelin‐reactive T cells can be isolated from control subjects as well as individuals who have MS. Experimental models of MS indicate that recently stimulated, myelin‐reactive T cells have greater encephalitogenic potential than resting T cells. Activation induces changes in T‐cell surface antigens that may distinguish previously stimulated, memory T cells from naive T cells. Therefore, we examined 108 myelin basic protein (MBP)‐reactive T‐cell lines from 7 MS and 8 control subjects to determine whether MBP‐reactive T cells originated in the memory T‐cell subset or in the naive subset. Isotypes of CD45 were used that designate memory or naive T cells. In subjects having MS, 84% of the MBP‐reactive T cells resided in the memory T‐cell subset. However, in control subjects, only 13% of MBP‐specific T cells originated from the memory T‐cell subset. This result suggests that a substantial proportion of MBP‐reactive T cells from some individuals with MS have been previously activated in vivo. This difference provides additional support for the hypothesis that myelin antigen–specific T cells are involved in the pathogenesis of MS. Ann Neurol 1999;45:33–39