A new accurate, fast catalytic method for the determination of trace amounts of mercury uptaken by plants is proposed. The method is based on the catalytic effect of mercury(II) on the rate of ligand-exchange reaction between ferrozine and hexacyanoferrate(II) whereby a coloured violet complex is formed which absorbs at 562 nm. The method allows the determination of mercury(II) in the range 0.05-4.0 ppm. The effect of pH, hexacyanoferrate, ferrozine, temperature, time of reaction and tolerance limit of many ions have been reported. The method is applied to the determination of mercury(II) uptaken by broad beans plants irrigated with water containing different amounts of mercury(II).