Large doses of ascorbic acid may mobilise Fe from Fe-binding proteins in vivo which in turn could catalyse lipid peroxidation, a process associated with degenerative diseases. This hypothesis was tested in vitro in the serum of Fe-loaded animals. Eighteen male guinea pigs weighing about 500 g on arrival were allocated to two groups of nine. Fe loading was induced in one group by two intraperitoneal injections of 200 mg iron dextran given on days 1 and 5. Blood (6 ml) was drawn from all animals on day 12 by cardiac puncture. Serum and LDL were separated. Serum was tested for loosely-bound Fe (bleomycin assay) and lipid peroxidation (thiobarbituric acid reactive substances (TBARS) assay) and LDL for susceptibility to in vitro oxidation (TBARS and conjugated diene assays). On day 12, another intraperitoneal injection of 200 mg iron dextran was given to the animals in the Fe-loaded group. On day 19, all animals were given 75 mg ascorbic acid by intraperitoneal injection. Blood (6 ml) was drawn 4 h later by cardiac puncture. Serum and LDL assays were repeated. Ascorbic acid increased loosely-bound Fe and in vitro oxidation in the serum from animals of the Fe-loaded group but not in the serum from animals of the control group. Susceptibility of LDL to in vitro oxidation increased after the ascorbic acid injection in the control group but there was no further increase in the Fe-loaded group. These data suggest that large doses of ascorbic acid promote Fe mobilisation and in vitro oxidation in the serum of Fe-loaded animals.