The BioBreeding/Worcester (BB/W) rat is a model of spontaneous autoimmune diabetes mellitus and lymphocytic thyroiditis. Additional features supporting an immunologic pathogenesis of the BB/W syndrome include the protective action of antilymphocyte serum, neonatal bone marrow transfusions, and neonatal thymectomy. To evaluate other manifestations of immune dysregulation, the BB/W colony was surveyed for the presence of autoantibodies to a variety of tissue and cell constituents. Anti-smooth muscle and anti-thyroid colloid antibodies were present with great frequency in diabetic animals as well as in normoglycemic offspring of diabetic parents. Anti-parietal cell antibodies were less frequent and islet cell cytoplasmic and adrenal antibodies were not detected. These data suggest that the underlying defect in the BB/W rat is more likely to be an abnormal immune regulatory system than antigenically altered target tissues (“altered self”) under attack by a normal immune surveillance system.