The role of Fas ligand-induced lymphocyte apoptosis in the development of insulitis was studied in nonobese diabetic (NOD) mice. Fas ligand with a molecular weight of 45 kD appeared in the pancreas of NOD mice during the onset of insulitis as demonstrated by western blot analysis. In situ DNA end-labeling (ISEL) of the pancreases of NOD mice demonstrated positive cells in the islets of Langerhans, with an age-dependent increase in the density and frequency of animals with islets containing ISEL-positive cells. In the pancreatic islets of BALB/c mice, no ISEL-positive cells were observed in any of the age groups studied. In ultrastructural analysis degenerating cells with condensed or fragmented nuclei and plasma membranes detached from neighboring cells were observed both in and around the islets. In some cases, these cells were being phagocytosed by the neighboring islet cells. Degenerating cells with characteristics of lymphocytes were seen in contact with healthy lymphocytes around the islets. Neutralizing Fas ligand antibodies affected 3H-thymidine incorporation of CD3+CD28+ pancreatic lymphocytes from 12- to 30-week-old NOD mice in one of three cultures. There was no difference in the effect of neutralizing Fas ligand antibodies between pancreatic and blood lymphocytes. The present results on an increase in density of apoptotic cells in pancreatic islets of NOD mice simultaneously with the onset of insulitis and appearance of Fas ligand suggest that the pancreas infiltrating lymphocytes may be destroyed by Fas ligand-induced apoptosis.