Systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD) are systemic autoimmune diseases that are characterized by the presence of autoantibodies reactive with U small nuclear RNP (snRNP) autoantigens. Both B and T cells are important in the pathogenesis of the disease, and T-and B-cell immunity against snRNP polypeptides have been shown to be linked in vivo. Currently, several alternative hypotheses for the pathogenesis of these diseases have been proposed. These include loss of tolerance, modified self-antigens, molecular mimicry and nondirected immune activation. To help distinguish between the various models of disease pathogenesis, we have characterized the T-cell receptor (TCR) CDR3 from a large panel of well-characterized human T-cell clones and lines specific for individual snRNP polypeptides. The results presented here reveal highly restricted TCR usage across patients by the snRNP-reactive T cells based on the deduced amino acid sequence of the CDR3 loop. These data support the hypothesis that T-cell responses against self antigens in SLE and MCTD are antigen driven and that there are a limited number of T-cell epitopes present on the snRNP autoantigens.