The different variables that affect the determination of recombinant human erythropoietin (r-HuEPO) by square wave voltammetry are studied. The method is based on oxidation of mercury from the electrode surface and following the reduction wave of the mercury bound to the thiol groups from four cysteine amino acids contained in the peptide backbone. The electrochemical behaviour of this system is evaluated by cyclic voltammetric studies. A 0.01 M phosphate buffer of pH 7.0 is used as supporting electrolyte and an accumulation potential of 170 mV (vs. AgCl/Ag) is applied during 300 s with continuous stirring at 700 'pm. The potential scan, without previous equilibration time, is realized with a square wave frequency f of 120 Hz, a scan increment AE of 4 mV and a square wave amplitude II of 50 mV. Under these conditions, the variation of i, with the r-HuEPO concentration is linear in the range from 0.040 to 0.400 pg 1-l. A limit of detection (3~) of 0.017 /.Lg 1-l and a limit of determination (10~) of 0.059 pg 1-l is obtained. The relative error of the proposed method is equal or less than 7.1%, and the relative standard deviation is equal or less than 5.3%. The proposed method is available to the determination of erythropoietin in urine.