Humoral immunity to bacterial antigens was investigated in 68 tissue typed and glucose tolerance tested first degree blood relatives of insulin dependent diabetics (IDD). The data were compared with those obtained in 60 IDDs and in 55 healthy controls. The prevalence of bacterial antibodies to E. coli, staphylococci, pertussis and diphtheria toxins were just slightly, but not significantly reduced in the blood relations compared with controls. Incidence of antibacterial antibodies was almost identical in blood relations with impaired and in those with normal glucose tolerance. By contrast, antibody formation to E. coli and staphylococci (p less than 0,0005, p less than 0,0005) respectively was significantly impaired in IDD. No correlation between genes of the major histocompatibility complex and humoral antibacterial immunity could be observed in IDD and blood relations. In conclusion, antibacterial antibody formation was found to be severely impaired in IDD patients but to be almost normal in blood relations of insulin dependent diabetics. These findings suggest that the humoral antibacterial immunodeficiency observed in IDD is a disease associated process probably independent of major histocompatibility complex linked genes.