A series of patients with juvenile onset diabetes (IDDM) and mature onset diabetes (NIDDM) have been typed for genetic variants of two sets of complement factors. For the HLA-linked C'2 system, there was found a significant increase of the C'2 2-1 type in IDDM compared with NIDDM patients or healthy controls. No such increase in any phenotype was observed for the non-HLA-linked C'6 system. These observations emphasize again the genetic distinction between IDDM and NIDDM, and the role of chromosome 6 in controlling susceptibility to the insulin-dependent form of the disease.