To study the cross-reactivity and functional properties oF murine T cells specific for myelin basic protein (MBP), a panel of 15 interleukin-2(IL-2)-releasing T-cell hybridomas was produced from SJL/J mice immunized either with human MBP or alternatively with a peptide corresponding to the known encephalitogenic sequence for SJL/J mice at positions 89-106. Hybridomas were I-As-restricted and activated by an MBP challenge as low as 20 nM. Cross-reactivity to other MBP indicated at least three immunodominant specificities for xenogeneic determinants, which could be further subdivided on the basis of antigen-independent reactivity to allogeneic stimulator cells. In addition, two self-specificities were demonstrated, one of which was to a determinant outside the 89-106 region. Irrespective of specificity pattern (self or foreign), all hybridomas effected antigen-dependent cytotoxicity of an antigen-presenting B-cell hybridoma (LS-102.9), which was mediated by cell contact or at close range. These findings suggest an approach to identifying new autoantigenic epitopes on MBP, and to studying T-cell-mediated effector pathways in myelin autoimmunity.