Multiple injections of streptozotocin to Charles River (CD-1) Laboratory mice resulted in a syndrome characterised by diabetes mellitus, insulitis and the induction of endogenous type C viruses in pancreatic beta cells. Within one week after the completion of five intraperitoneal injections of streptozotocin, the CD-1 mice exhibited irreversible hyperglycaemia and insulinopaenia. Light microscopic studies of pancreata from mice sacrificed at this time demonstrated insulitis and beta cell necrosis. Electron microscopic studies revealed spherical and atypical cylindrical type C viruses and occasional clusters of intracisternal type A viruses exclusively within beta cells. To clarify the identification of the type C viruses and their role in the genesis of the insulitis, type C virus specific antigens were identified within islet cells by immune fluorescence at various intervals after streptozotocin administration. Immune fluorescence studies demonstrated the presence of type C virus antigens within islets from streptozotocin treated mice but not in buffer-injected controls. Time course studies suggested that type C virus induction may precede the appearance of insulitis by two days and that insulitis is consistently accompanied by the presence of virus positive islet cells.