The following immunological functions were studied in a case of insulin dependent diabetes mellitus with Graves' disease: (1) Lymphocyte subpopulations, (2) mitogen response, (3) immunoglobulin producing cells by the plaque forming cell assay, (4) cell-mediated cytotoxicity, and (5) natural killer activity were normal. The patient lacked antibody-dependent cell-mediated cytotoxicity. No conclusion could be drawn as to whether effector cells mediating natural (NK cell) and antibody-dependent cell-mediated cytotoxicity (K cell) are identical or different. However, the existence of such a case strongly suggests that the K cell is distinct from the NK cell. To resolve the question of whether or not a loss of antibody-dependent cell-mediated cytotoxicity activity plays an important role in the development of diabetes mellitus and Graves' disease, further studies on a large number of cases are necessary.