Biological characteristics of diabetogenic and non-diabetogenic variants of encephalomyocarditis (EMC) virus have been studied in tissue culture and in animal model systems. The underlying molecular mechanisms responsible for these properties are currently under investigation. The interaction of different environmental agents (e.g. viruses and chemicals) in the development of diabetes in experimental animals has also been studied. Cytopathology in human islet tissue as a result of acute viral infections has been the subject of a retrospective study. Autoimmunological phenomena associated with juvenile-onset diabetes mellitus have been studied. This has included the identification and partial functional characterization of cytotoxic antibodies to islet cells in sera from diabetics and their first-degree relatives. An animal model system for virus-induced autoimmunity and diabetes has also been developed.