In humans and non‐obese diabetic mice (NOD), insulin‐dependent diabetes mellitus (IDDM) results from a spontaneous T cell‐dependent autoimmune destruction of insulin‐producing pancreatic β cells. Previous data suggest that a delicate balance between autoaggressive T cells and suppressor‐type immune phenomena determine whether expression of autoimmunity is limited to insulitis or progresses to IDDM. To resolve the cellular basis of this intricate network of pathogenic CD4⁺ and CD8⁺ T cells and the role of T cells in suppressive immune phenomena, T cell clones were propagated directly from islets of NOD mice at the onset of insulitis. Insofar as insulitis, but not IDDM, is universal in NOD mice, we have screened for the in vivo effects of the islet‐infiltrating T cell clones upon expression of IDDM, not insulitis. A CD4⁺ T cell clone, IS‐3S7D, proliferates in response to islet antigen(s) and its transfer into prediabetic NOD mice promotes the rapid onset of IDDM. An interleukin 2 (IL 2)‐dependent noncytolytic, V_β11⁺ CD8⁺, T cell clone, IS‐2.15, prevents an accelerated onset diabetes in two distinct models. The present study, which documents the presence of CD4⁺ diabetogenic T cell clones and CD8⁺ T cell clones that dampen autoimmunity, gives tangible evidence that opposing autoimmune processes may determine whether an autoimmune‐prone host develops frank disease.