IMMUNIZATION with myelin basic protein (MBP) induces experimental allergic encephalomyelitis (EAE), a prototype of CD4⁺ T-cell mediated autoimmune disease. In rodents, MBP-reactive T-cell clones are specific for a single, dominant determinant on MBP and use a highly restricted number of T-cell receptor genes^1–3. Accordingly, EAE has been prevented by various receptor-specific treatments^1–8, suggesting similar strategies may be useful for therapy of human autoimmune disease. Here we report that in (SJL x B 10.PL)F₁ mice, immune dominance of a single determinant, MBP: Ac1–11, is confined to the inductive phase of EAE. In mice with chronic EAE, several additional determinants of MBP in peptides 35–47, 81–100 and 121–140 recall proliferative responses. Most importantly, reactivity to the latter determinants was also detected after induction of EAE with MBP peptide Ac1–11 alone; this demonstrates priming by endogenous MBP determinants. Thus, determinants of MBP that are cryptic^9–11 after primary immunization can become immunogenic in the course of EAE. Diversification of the autoreactive T-cell repertoire due to 'determinant spreading' has major implications for the pathogenesis of, and the therapeutic approach to, T-cell driven autoimmune disease.