Oral administration of antigen is a long-recognized method of inducing systemic immune tolerance. In animals with experimental autoimmune disease, a major mechanism of oral tolerance involves the induction of regulatory T cells that mediate active suppression by secreting the cytokine TGF-beta 1. Multiple sclerosis (MS) is a presumed T cell-mediated Th1 type autoimmune disease. In this paper we investigated, in patients with MS, whether oral myelin treatment (myelin containing both MBP and PLP) induced antigen-specific MBP-or PLP-reactive T cells that were either Th2-like (secreted IL-4 or TGF-beta 1), or alternatively whether Th1 type sensitization occurred as measured by IFN-gamma secretion. Specifically, 4,860 short-term T cell lines were generated to either MBP, PLP or TT from 34 relapsing-remitting patients with MS; 17 were orally treated with bovine myelin daily for a minimum of two years as compared to 17 non-treated patients. We found a marked increase in the relative frequencies of both MBP-and PLP-specific TGF-beta 1 secreting T cell lines in the myelin-treated MS patients as compared to non-treated MS patients (MBP, p< 0.001; PLP, p< 0.003). In contrast, no changes in the frequency of MBP-or PLP-specific IFN-gamma or TT-specific TGF-beta 1 secreting T cells were observed. These results suggest that the oral administration of antigens generates antigen-specific TGF-beta 1 secreting T cells of presumed mucosal origin that may represent a distinct cytokine-secreting lineage of T cells (Th3). Since, in animal models, antigen-specific TGF-beta 1 secreting cells localize to the target organ and then suppress inflammation in the local microenvironment, oral tolerization with self-antigens may provide a therapeutic approach for the treatment of cell-mediated autoimmune disease which does not depend upon knowledge of the antigen specificity of the original T cell clone triggering the autoimmune cascade.