Multiple sclerosis (MS) is a demyelinating disease of the central nervous system with a presumed autoimmune pathogenesis involving autoantigen-specific CD4+ T cells and cytokines. A similar frequency of T cells responding to myelin basic protein (MBP), a putative target in MS, has been observed in MS patients and controls. To dissect the differences between MBP-specific T cells in patients and controls, we have analyzed the cytokine secretion profile of such autoreactive T cells. MBP-specific T cell clones (TCC) were isolated from the peripheral blood of MS patients and controls by limiting dilution. Expression of mRNA for interferon-gamma (IFN-gamma), interleukin (IL)-4, IL-10, tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta (TGF-beta) was assessed by polymerase chain reaction whereas secretion of cytokine protein was measured by ELISA. MBP-specific TCC exhibited a heterogeneous cytokine secretion profile with clones displaying Th1, Th2 and Th0 phenotypes. A significant difference in the distribution of the cytokine profile was noted between MS patients and controls. Although the frequency of Th1 secreting MBP-reactive TCC was similar between MS patients and controls, stable MS patients had a significant association with the Th0 phenotype whereas healthy individuals were associated with the Th2 phenotype. In comparison to control TCC, MBP-specific TCC from MS patients secreted increased amounts of IFN-gamma, IL-4 and IL-10 and decreased quantities of TGF-beta. Thus, these studies suggest that there is a dysregulation in the balance between pro-inflammatory Th1 and anti-inflammatory Th2 cytokines in MS. It appears that the presence of Th1 secreting autoreactive T cells in healthy individuals may be counterbalanced by the presence of cells secreting Th2 cytokines and by the augmented production of the immunosuppressive cytokine TGF-beta, whereas in MS there is a decrease in these anti-inflammatory agents.