A group of 67 juvenile insulin dependent diabetic patients and their 167 healthy first degree blood relatives were HLA-typed. In the patients the frequency of HLA-A9 and B8 antigens was significantly increased as compared to healthy controls, while in the family members only the presence of HLA-B8 was significantly increased. All diabetics carrying HLA-B8 antigen had frequently higher 125I-insulin-antibody complex levels than those lacking the antigen. Prevalence of some autoantibodies to human thyroglobulin, microsomal thyroid and antigastric mucosa antigen were investigated and compared to healthy controls. Increased antibody titres were more frequent in diabetics and their blood relatives than in the healthy controls, and more frequent in those carrying the HLA-B8 antigen than in those lacking it.