The nature of non-transferrin-bound iron in the plasma or serum of iron-overloaded hemochromatosis patients was studied by high performance liquid chromatography (HPLC) and high resolution nuclear magnetic resonance (NMR). 500-MHz proton Hahn spin-echo NMR spectra of plasma or serum, combined with the use of the iron chelator desferrioxamine, suggests complexation of iron ions with citrate and a possible involvement of acetate. Addition of FeCl3 to hemochromatosis samples broadened the NMR signals from citrate. HPLC analysis rigorously confirmed the presence of an iron-citrate complex in ultrafiltrates of plasma or serum studies with added FeCl3 or desferrioxamine supported this conclusion. It is proposed that non-transferrin-bound iron in the plasma of iron-overloaded patients exists largely as complexes with citrate and possibly also as ternary iron-citrate-acetate complexes. The presence of such complexes would account for the ability of non-transferrin-bound iron to be measurable by the bleomycin assay and for its rapid clearance from the circulation by the liver.