We isolated CD4+ and CD8+ T cell clones from pancreatic islets of non-obese diabetic (NOD) mice and studied their interactions with pancreatic islets, in culture. The three CD4+ T cell clones proliferated when cultured with islet cells from NOD, BALB/c, or C57BL/6 (B6) mice. For proliferation to the allogeneic islets, however, APC from NOD mice were required in the culture. Two of the clones also produced IFN-gamma upon culture with NOD islet cells. The Ag from islet cells responsible for T cell stimulation were not released into the supernatant but were cell associated. Paraformaldehyde treatment of islet cells, in fact, preserved their antigenicity. The fixed islet cells could present Ag to CD4+ T cell clones, provided live, syngeneic APC were added to the culture. We conclude from these experiments that islet cells donate Ag to the APC for presentation and that the function of APC is to process the Ag. The two CD8+ T cell clones proliferated and released IFN-gamma upon reaction with islet cells from either NOD or BALB/c but not B6 mice. The CD8+ T cell clones also reacted with the insulinoma NIT-1 cell line, derived from NOD mice. Fixation of NIT-1 cells did not impair recognition when live APC were present in the culture. In this case, however, the APC could be allogeneic. We conclude that CD8+ T cells directly recognized a MHC class I-restricted Ag on target cells, but needed the costimulatory effect of APC. We also found that CD8+ T cells killed islet cells. Two of the CD4+ T cell clones produced diabetes when transferred into male, irradiated NOD mice. For optimal transfer of disease, the CD4+ T cell clones had to be co-injected with CD8+ T cells from NOD diabetic mice. The two CD8+ T cell clones did not transfer disease.