Oral tolerance is defined by immune unresponsiveness after oral administration of soluble antigens and by antigen-specific inhibition of peripheral immune responses induced by prior antigen feeding. The aim of this study was to investigate the implication of the major histocompatibility complex (MHC) class II presentation pathway to CD4+ T cells in oral tolerance of contact sensitivity (CS) to the hapten dinitrofluorobenzene (DNFB). We used MHC class II knockout (ABº/º) and invariant chain knockout (Iiº/º) mice, which have, respectively, a total or partial defect in class II-restricted activation of CD4+ T cells, as well as normal C57BL/6 mice depleted of CD4+ T cells by injection of a specific antibody. Intragastric administration of DNFB prior to skin sensitization induced specific inhibition of contact sensitivity to DNFB in Aβ+/º and Ii+/º heterozygotes comparable to that observed in C57BL/6 mice. In contrast, no oral tolerance was observed in either MHC class II-deficient Aβº/º and Iiº/º homozygote mutants or in syngeneic anti-CD4-depleted C57BL/6 mice. Moreover, a single oral administration of DNFB, without skin sensitization, could prime Aβº/º, Iiº/º as well as anti-CD4-depleted C57BL/6 mice for DNFB-specific CS. These findings demonstrate that the class II/CD4 pathway is involved in oral tolerance manifested both as the inhibition of CS by hapten feeding prior to skin sensitization, and as immune unresponsiveness of normal mice to oral administration of hapten. Furthermore, our data provide evidence that a single oral feeding with DNFB is able to prime mice for hapten-specific CS, provided that the class II/CD4 pathway is bypassed.