The binding of Fe³⁺ to red cell membranes was studied in a system in which lipid peroxidation was proportional to Fe³⁺ concentration. Binding of Fe³⁺ was evaluated by labeling with ⁵⁹FeCI₃ and measurement of NMR water-proton relaxation times. Labeling with ⁵⁹Fe showed that 95% of the Fe³⁺ was membrane bound at 100μM FeCI₃ in a 1.5 mg proteiniml membrane suspension. Both spin-lattice (T₁) and spin-spin (T₂) relaxation times decreased with increasing Fe³⁺ concentration. Addition of red cell membrane suspensions largely prevents the Fe³⁺ effect on relaxation times. Charge transfer to Fe³⁺ may occur at the membrane binding site with resultant decrease in the Fe³⁺ effect on water-proton relaxation times. These studies support the hypothesis that Fe³⁺ binds to the membrane and generates free radicals at the binding site.