Serum iron concentrations greater than 90 mumol/L were measured in samples from hemodialysis patients by a Ferrozine dye-binding method. Reanalysis by coulometry showed these results to be spuriously high. Turbidity resulting from precipitation of fibrinogen was identified as the source of the interference. High concentrations of heparin enhanced the effect. We conclude that the persistence of fibrinogen in serum samples from patients treated with anticoagulants is a potential interference in Ferrozine dye-binding methods performed without prior deproteinization. Adding guanidine.HCl to the acid buffer reagent is a simple way to eliminate this interference.