The differential pulse cathodic stripping voltammetry has been employed to determine the concentration of 5-flourouracil (5-Fu) in serum.The interferences of electroactive substanees in serum can be eliminated by trichloroacetic acid. It is shown that 5-Fu can combine with Hg(Ⅱ) generated by anodic polarization forming an insoluble complex on electrode surface, The Hg-5-Fu complex is deemed to be a macromolecule with chain structure.