Abstract: Diphtheria toxoid was denatured in 0.1M NH₄Cl-NH₃H₂O basal solution with pH 9.4,yielding a cathodic and an anodic polarographic peaks at -1.52 and -1.45V(vs.SCE),respectively.The cathodic and anodic peak heights varied linearly with diphtheria toxoid concentration over the range of 2.5 to 180μg/ml by single sweep polarography,and diphtheria toxoid even as low as 0.3μg/ml could be detected by differential pulse polarography.The decreased peak current value of diphtheria toxoid due to immunochemical binding reaction was proportional to diphtheria antitoxin in the range of 2.5×10^-6 to 5×10⁵ IU/ml,which determined diphtheria antitoxin level homogeneously.The assays are sensitive,specific,reproducible and simple in operation.The anodic peak was appl.ied to the determination of diphtheria toxoid in the triple vaccine.