Peptides binding to a particular class II major histocompatibility complex (MHC) molecule can inhibit the activation of T cells by other peptides binding to the same molecule, a phenomenon termed class II MHC blockade. All class II-binding peptides exert MHC blockade in vivo in depot form with adjuvant, and some also retain their blocking properties in soluble form. We demonstrate here that soluble peptides, when used at doses causing short-term MHC blockade, can also induce long-term antigen-specific T cell tolerance to themselves. The tolerogenicity of soluble peptides correlates with their antigenicity in adjuvant, but it is not necessarily related to their capacity to act as class II blockers in vivo. The tolerant state is manifested in a decreased production of both T helper cell 1 (Th1)-type and Th2-type lymphokines, and it cannot be reversed by interleukin-2. Once T cells are primed with a peptide in complete Freund's adjuvant, they are resistant to tolerization with the same peptide applied in soluble form. Tolerance induction is partially impaired in B cell-deficient mu MT-/- mice, suggesting a role for B cell antigen presentation in this process. The results suggest that the potential immunogenicity of class II MHC blockers could be circumvented by choosing a tolerogenic mode of application.