The kinetics of the uptake of Fe(II)-histidinate, a known promoter of lipid peroxidation, into Ehrlich ascites tumor (EAT) cells and the intracellular binding of iron were studied in vitro. EAT cells (27.10(6)/ml) were incubated in Hanks' balanced salts solution at 37 degrees C for various time intervals in the presence of FeSO4 (1 mM) and L-histidine (10 mM). Total iron was determined by the 1,10-phenanthroline/ascorbate method and ferric iron by reaction with 5-sulfosalicylic acid; the difference was ascribed to ferrous iron. Total iron decreased rapidly in the medium (242 nmol within the first 10 min), and a corresponding increase of total iron (saturation value 376 nmol after 60 min) was determined within the cells, after the cellular proteins had been solubilized with 6 M urea. In the absence of EAT cells, Fe(II)-histidinate was readily oxidized to Fe(III)-histidinate by oxygen, but this reaction was strongly retarded by the tumor cells. The uptake of iron histidinate occurred in the oxidized state, while an uptake of ferrous iron could not be proven unambiguously. When EAT cells were saturated with iron, it was found that 93% of intracellular iron was bound to water-insoluble proteins and 7% was associated with soluble proteins, while no unbound iron was detectable by the method used. It was concluded that, despite the high uptake of total iron, only a very small portion of the intracellular iron was available as a redox catalyst for lipid peroxidation.