The chromogenic substrates ferrozine and ferene were compared to bathophenanthroline disulphonic acid for the measurement of iron concentrations in aqueous and serum samples in an assay based on that of the Iron Panel of the International Committee for Standardisation in Haematology. Ferrozine and ferene were more sensitive than bathophenanthroline. Copper at physiological concentrations in plasma caused only minimal positive interference with all three chromogenic substrates when thioglycollic acid was used as the reducing agent, but when ascorbic acid was used significant positive interference occurred with ferrozine and ferene. Interference due to contaminating haem was comparable with all agents. Bilirubin and carotene produced no interference. Profound reductions in colour development were noted with EDTA plasma.