Insulin-specific T cells have been found to be present in high frequency among nominally islet-cell-specific T cells in the islet infiltrates that accumulate in NOD mice. In a previous report in which clones obtained from 7- and 12-week-old mice were examined, we identified a 15-residue peptide of the B chain as the dominent epitope for this response. Despite the fact that the response to insulin appears to be directed toward this single peptide, diverse TCR Vβ usage was observed. That insulin-specific T cells contribute to β cell damage is suggested by the fact that all clones tested could mediate β cell destruction upon adoptive transfer. In the present report we extend this examination of insulin-specific T cells to lines and clones established from mice ranging in age from 4-12 weeks. These clones were found to be very similar to those from 7- and 12-week-old mice. The response was directed to the same peptide and most were found to produce IFNγ, but none produced IL-4.