A new fading spectrophotometric method for determining trace heparin is proposed in the present paper. In pH 3.0 Britton-Robinson (B-R) buffer solution, brilliant cresyl blue can react with heparin resulting in the decrease of absorbance of BCB at 592 nm. The conditions for the interaction were optimized, and the interferences of coexisting substances were investigated. Under the optimal conditions the reduction of the absorbance is proportional to the concentration of heparin in the range of 0.6-6.0 mg x L(-1) with the detection limit of 0.173 mg x L(-1). The molar absorptivity of the method was calculated to be 1.03 x 10(6) L x mol(-1) x cm(-1). The method has been applied to determine the heparin sodium injection solution with satisfactory results.