Ceruloplasmin (Cp) is an acute-phase-responsive oxidase enzyme. Prior reports suggest that Cp is increased in diabetes mellitus, perhaps reflecting greater oxidant stress. However, the situation in insulin-dependent diabetes mellitus (IDDM) per se remains unclear. Furthermore, vitamin C can interfere with one indirect assay for Cp, and vitamin C metabolism is altered in IDDM. We measured Cp levels by both a direct radial immunodiffusion (RID) assay and an indirect oxidase assay in 10 subjects with IDDM and 10 nondiabetics, both at baseline and after 30 days of vitamin C supplementation (100 or 600 mg daily, five subjects per group). Plasma copper level was measured independently also. Our data show that circulating levels of Cp are significantly increased in IDDM subjects as a group, and specifically that Cp is abnormally high in a subset of IDDM individuals. Vitamin C supplementation at either dose interfered with the oxidase assay for Cp in both groups, but vitamin C did not alter the RID assay. The observed increase in plasma copper suggests that circulating holo-Cp is increased. The finding of increased Cp in some individuals with IDDM supports the hypothesis of increased oxidant stress as a variable factor in the spectrum of chronic complications in diabetes. Measurements of Cp level by the oxidase assay must be considered unreliable for subjects taking vitamin C supplements of > or = 100 mg/d.