Nonobese diabetic (NOD) mice develop an autoimmune form of diabetes, becoming hyperglycemic after 3 months of age. This process was accelerated by injecting young NOD mice with CD4⁺ islet-specific T cell clones derived from NOD mice. Overt diabetes developed in 10 of 19 experimental animals by 7 weeks of age, with the remaining mice showing marked signs of the disease in progress. Control mice did not become diabetic and had no significant pancreatic infiltration. This work demonstrates that a CD4 T cell clone is sufficient to initiate the disease process in the diabetes-prone NOD mouse.