We have characterized the major histocompatibility complex (MHC) specificity of autoreactive T cell clones arising from diverse donors after immunization with different antigens. The MHC fine specificity of autoreactive T cells for unique F1 hybrid determinants of BALB.K X BALB.B F1, and for the mutant I-Ab determinants of the B6.C-H-2bm12 (bm 12) strain is similar to that previously described for antigen-specific T cells. We find, furthermore, that the MHC specificity of autoreactive T cell clones selected from primed populations grown in the absence of Con A-stimulated supernatant factors reflects the predominant MHC restriction specificity of T cells specific for the immunogen. Thus, I-E subregion-specific autoreactive T cells are detected at a much higher frequency after immunization with the I-E-restricted antigen, GL (terpolymer of glutamic acid, lysine, and phenylalanine), than with the predominantly I-A-restricted antigen, keyhole limpet hemocyanin (KLH). These experiments strongly suggest that some autoreactive T cells are derived from antigen-stimulated precursors. This result contrasts with that obtained when autoreactive T cells are selected in bulk cultures, or in the presence of exogenous T cell factors. We conclude that, under optimal conditions, most autoreactive T cells are recruited from a relatively stable pool of predominantly I-A-specific precursors. Autoreactive precursors in this pool might themselves derive from previous antigenic stimulation, or be of independent origin.