Oral administration of myelin basic protein (MBP) is an effective way of suppressing experimental autoimmune encephalomyelitis (EAE). We have previously shown that such suppression is mediated by CD8+ T cells, which adoptively transfer protection and suppress immune responses in vitro. In the present study we have found that modulator cells from animals orally tolerized to MBP produce a suppressor factor upon stimulation with MBP in vitro that is specifically inhibited by anti-transforming growth factor beta (TGF-beta) neutralizing antibodies. No effect was observed with antibodies to gamma interferon, tumor necrosis factor alpha/beta, or indomethacin. In addition, the active form of the type 1 isoform of TGF-beta 1 (TGF-beta 1) can be directly demonstrated in the supernatants of cells from animals orally tolerized to MBP or ovalbumin after antigen stimulation in vitro. Antiserum specific for TGF-beta 1 administered in vivo abrogated the protective effect of oral tolerization to MBP in EAE. Furthermore, injection of anti-TGF-beta 1 serum to nontolerized EAE animals resulted in an increase in severity and duration of disease. These results suggest that immunomodulation of EAE induced by oral tolerization to MBP and natural recovery mechanisms use a common immunoregulatory pathway that is dependent on TGF-beta 1. Implications of such an association are of therapeutic relevance to human autoimmune diseases and may help to explain one of the mechanisms involved in the mediation of active suppression by T cells.