To examine the hypothesis that a single initiating antigen was recognized by a monoclonal T cell population leading to subsequent inflammatory insulitis in nonobese (NOD) mouse islets, we examined the T cell receptor (TCR) Vβ repertoire of islet-infiltrating T cells in very young (2-week-old) NOD mice. In independent experiments, we repeatedly identified one monoclonal TCR Vβ8.2 gene product expressed by T lymphocytes infiltrating the islets of NOD mice at 2 weeks of age. The resultant inflammatory response quickly obscures the monoclonal nature of the initiating event. These data suggest that autoimmune diabetes in NOD mice may be initiated by recognition of a single autoantigen.