Aims/hypothesis. A mechanism implicated in pancreatic islet beta-cell destruction in autoimmune diabetes is the binding of the Fas ligand (FasL) on T cells to Fas receptors on beta cells, causing their destruction. Evidence for this mechanism is, however, controversial. The aim of this study was to find whether the Fas ligand contributes to beta-cell death in autoimmune diabetes. Methods. We transplanted syngeneic islets under the renal capsule in non-obese diabetic (NOD) mice and treated the mice with a neutralizing monoclonal antibody to the Fas ligand. Survival of beta cells in islet grafts and phenotypes of graft-infiltrating cells were investigated. Results. We found 58 % (7 of 12) of mice treated with anti-Fas ligand antibody were normoglycaemic at 30 days after islet transplantation compared with none (0 of 9) of the mice treated with control antibody. Immunohistochemical analysis of islet grafts showed that infiltration of leucocytes (CD4⁺ T cells, CD8⁺ T cells, macrophages and neutrophils) and apoptosis of beta cells in the grafts was significantly decreased in mice treated with anti-Fas ligand antibody. Expression of proinflammatory cytokines (interleukin 1 alpha, tumour necrosis factor alpha and interferon gamma) was not different in islet grafts of mice treated with anti-Fas ligand and control antibodies. Conclusion/interpretation. These findings indicate that Fas ligand-mediated mechanisms play a major part in promoting leucocytic infiltration of islets and beta-cell destruction in autoimmune diabetes. [Diabetologia (2000) 43: 1149–1156]