Antigen-driven tolerance is an effective method of suppressing cell-mediated immune responses. We have previously shown that oral administration of myelin basic protein (MBP) suppresses experimental autoimmune encephalomyelitis (EAE) when it is actively induced by MBP emulsified in complete Freund's adjuvant. In order to further study antigen-driven tolerance in this model, we investigated the effect of oral tolerization on adoptively transferred EAE and compared oral tolerance to intravenously (i.v.) administered MBP in both actively induced EAE and adoptively transferred EAE. Although orally tolerized animals were not protected from adoptively transferred EAE, spleen cells from orally tolerized animals suppressed adoptively transferred EAE when co-transferred with encephalitogenic cells or when injected into recipient animals at a different site at the time encephalitogenic cells were transferred. This suppression was mediated by CD8+ T cells, correlated with suppression of DTH responses to MBP, and was associated with decreased inflammation in the spinal cord. Unlike oral tolerization, spleen cells from i.v. tolerized animals did not suppress adoptively transferred EAE when co-transferred with encephalitogenic cells although i.v. tolerized animals were protected from adoptively transferred EAE. MBP peptides were then utilized to further characterize differences between i.v. and oral tolerization in the actively induced disease model. Both orally and intravenously administered MBP suppressed actively induced EAE. However, EAE was only suppressed by prior i.v. tolerization with the encephalitogenic MBP peptide 71-90, but not with the non-encephalitogenic peptide 21-40, whereas prior tolerization with 21-40 did suppress actively induced EAE when administered orally. These results suggest a different mechanism of tolerance is initiated by oral vs. intravenous administered antigen. Specifically, oral tolerization suppresses primarily by the generation of active suppression whereas the dominant mechanism of suppression associated with i.v. tolerization appears most consistent with the elicitation of clonal anergy.