Data from our laboratory are reviewed showing that both cell-mediated cytotoxicity, complement-dependent antibody-mediated cytotoxicity, antibody-dependent cellular cytotoxicity, and complement-augmented antibody-dependent cellular cytotoxicity may provide mechanisms which kill pancreatic islets during pathogenesis of insulin-dependent (type I) diabetes. Xenogenic test systems employing dispersed rat islet target cells were employed. Most of the findings were reversible during clinical remission of diabetes.