Five hundred and ninety-nine patients with insulin-dependent (Type 1) diabetes mellitus were typed for Bf (factor B) polymorphism, and 318 of them for HLA-A, B and C antigens. Bf and HLA antigen frequencies were compared with those in 536 normal controls. A significant positive association between Type 1 diabetes and the rare factor B variant BfF1 was found (p less than 10(-3)), but this was present only in patients aged under 16 years at onset of the disease (p less than 0.005). There was a strongly positive linkage disequilibrium between BfF1 and HLA-B18 in the diabetic patients. This, too was especially pronounced in the juvenile onset cases, in whom it was significantly stronger than in controls (p less than 10(-3)). The known positive associations between Type 1 diabetes and HLA-B8, -B15 and -Cw3 were confirmed.