It has been proposed that the development of insulin-dependent diabetes is controlled by the T helper 1 (T(H)1) versus T(H)2 phenotype of autoreactive T-H cells : T(H)1 cells would promote diabetes, whereas, T(H)2 cells would actually protect from disease. This proposition was tested by establishing cultures of T(H)1 and T(H)2 cells that express and identical diabetogenic T cell receptor and comparing their ability to initiate disease in neonatal nonobese diabetic mice. T(H)1-like cells actively promoted diabetes; T(H)2-like cells invaded the islets but did not provoke disease-neither did they provide substantial protection.