This study describes a simple,rapid and selective catalytic kinetic spectrophotometric method for the determination of 6-monoacetylmorphine (6-MAM) as major metabolite of heroin in biological samples.The method is based upon the catalytic effect of 6-MAM on the oxidation of Janus Green by bromate in acid media.The reaction was followed spectrophotometrically by measuring the decrease in absorbance of Janus Green at 618 nm.The dependence of sensitivity on the reaction variables was studied.Under optimum conditions,two linear calibration curves over the range 0.1-1.0 μg mL-1 and 1.0-34.0 μg mL-1 of 6-MAM were obtained.The detection limit was 1.2 × 10-2 μg mL-1 of 6-MAM.The relative standard deviations for six replicate determinations of 0.8 and 5.0 μg mL 1 of 6-MAM were 1.4 and 1.1％ respectively.The effect of various species commonly associated with heroin in real samples was also investigated.The proposed method was successfully applied in human urine and serum samples with satisfactory results.