A method for the determination of a-1-fetoprotein (AFP) in human serum by using a horseradish peroxidase (HRP) label in an enzymelinked immunosorbent assay (ELISA) is proposed. The method is based on the electrochemical determination of enzymatic reaction product with differential pulse voltammetry at a gold disk electrode. The assay consists of two successive steps. The ®rst step is a conventional HRP-mediated ELISA for the formation of electroactive 2,2 H -diaminoazobenzene by means of the o-phenylenediamine2 2H 2 O 2 2 2HRP system. At the second step, 2,2 H -diaminoazobenzene exhibits a sensitive voltammetric response at À0.19 V in pH 2.0 PBS. The peak current is proportional to the concentration of AFP in the range of 0.5±400 ngymL (R 0.9993) under optimum conditions. The sensitivity of this method is higher than that of the traditional spectrophotometric ELISA procedure. The proposed method has been applied to the clinical determination of AFP in human serum with satisfactory precision and accuracy.