We have been investigating the mechanisms by which exogenous protein Ags activate CD8+ T cells. Previously, we have shown that OVA primed CTL precursors in vivo if administered as an emulsion with an adjuvant such as CFA. Such CTLs inhibit development of Ab responses when adoptively transferred into syngeneic mice. Thus, CD8+ CTL are immunosuppressive. These studies were initiated to determine whether CD8+ suppressor T cells were cytolytic T cells. Oral administration of protein Ags is a well-established method for inducing tolerance in humoral and delayed hypersensitivity responses which is associated with development of CD8+ suppressor T cells. OVA was chosen as a model Ag because it has been used extensively in oral tolerance studies, and target cells expressing the OVA gene are well characterized. Our data show that multiple, intragastric doses of native OVA inhibited priming of CD8+ CTL precursors and also inhibited CD4+ T cells and Ab responses. Oral OVA inhibited CTL priming in mice immunized with OVA-loaded EL4 cells, OVA-expressing transfectants, or OVA in CFA. The observed tolerance is specific to the orally administered Ag. Although oral OVA did not prime CTL precursors, it did activate spleen cells that transferred unresponsiveness to naive syngeneic mice. Suppression was mediated by CD4-CD8+ T cells. These CD8+ suppressor T cells were phenotypically distinguished from CTL by reactivity with a mAb that recognizes activated suppressor T cells.