Experimental allergic encephalomyelitis (EAE) is a model of antigen-specific T cell-mediated autoimmune disease. The alpha-acetylated, NH2-terminal nine amino acids (1-9NAc) of myelin basic protein (MBP) represents the dominant T cell epitope for the induction of EAE in the B10.PL (H-2u) strain. We tolerized neonatal B10.PL mice to 1-9NAc and studied the proliferative responses to this peptide and to whole MBP. Mice exposed to 1-9NAc in the neonatal period were tolerant to subsequent challenge at the proliferative T cell level. Similarly, in the 1-9NAc-tolerant group, both the incidence and severity of 1-9NAc induced EAE were greatly reduced. The fact that we were able to tolerize mice normally responsive to MBP suggests that this self antigen is sequestered (within the central nervous system) and hence tolerance to it is not normally induced. No significant difference in disease incidence was seen in response to rat MBP between control animals and 1-9NAc-tolerized mice (50% in both groups), demonstrating the presence of at least one additional encephalitogenic determinant elsewhere on the molecule. We have successfully prevented disease induction by peptide-induced tolerization. Tolerance induction by peptides provides a new and specific strategy in the prevention of autoimmunity. However, it will be clearly necessary to fully define all epitopes potentially capable of inducing pathogenic T cells to ensure complete and effective therapy of T cell-mediated autoimmune disease.