Nonobese diabetic (NOD) mice display a syndrome with dramatic clinical and pathological features similar to Type 1 diabetes in man. Mononuclear cells intensively infiltrate the pancreas (insulitis), mostly T cells taking part. However, the functional role and specificity of these cells are currently uncertain. We investigated the response of splenic T cells from NOD mice to islet cells, using interleukin 2 (IL-2) production and cell proliferation. Splenic T cells from NOD mice responded with IL-2 production and proliferation when both islet cell antigens from NOD mice and mitomycin C-treated spleen cells (source of antigen-presenting cells) from NOD mice or major histocompatibility complex (MHC)-compatible ILI mice were present. Splenic T cells could produce IL-2 in response to islet cells from sources other than the NOD mouse, but could not produce significantly this lymphokine in response to submandibular gland, gastric mucosal, liver, spleen, and ovarian cells from NOD mice. NOD T cells produced this antigen-specific response first when the mice were 8 weeks old, the response grew stronger until 20 weeks of age and then tapered off. The present study indicates the presence of T cells specific for islet cell antigens in the spleen of NOD mice and suggests that the antigen-specific T cell response increases in parallel with the development of insulitis.