In the nonobese diabetic (NOD) mouse, susceptibility to insulin-dependent diabetes mellitus is in part controlled by a single expressed class II major histocompatibility complex (MHC) molecule, I-Ag7. This molecule probably exerts its control through the representation of a self-peptide, derived from an unknown beta cell antigen, leading to T cell activation and eventual islet destruction. In this paper, synthetic peptides have been used to compete for binding to the I-Ag7 molecule in an attempt to suppress the autoimmune response. The administration of an I-Ag7-binding immunogenic peptide, lambda repressor (cI) 12-26, in a water and oil emulsion (incomplete Freund's adjuvant) can prevent the transfer of IDDM into irradiated recipients by spleen cells from diabetic donors. Nonbinding, nonimmunogenic peptides have no effect in this situation. However, the immune response to the "blocking" peptide in these experiments was a complicating factor in interpreting the results. To establish that the effect was at the level of competition for MHC binding, two additional approaches were tried. First, tolerance was induced to the immunogenic peptide, cI 12-26, before using it to "block" disease. Tolerance abolished the effect on diabetes transfer. Second, an effort was made to identify peptides that were nonimmunogenic but that bound to I-Ag7. Such a peptide, mouse prostatic secretory glycoprotein precursor 63-76, had no effect on the incidence of transferred disease. We conclude that the "blocking" effects seen in initial experiments in the NOD mouse were not caused by blockade of MHC presentation, but by other unknown effects related to the immunogenicity of the "blocking" peptide.