Xanthine oxidase at pH values greater than its isoelectric point has been immobilized electrochemically in the polypyrrole film, which provides evidence for the doping mechanism of conducting polymers. The polypyrrole xanthine oxidase electrode formed in this manner has the kinetic behaviour of a free enzyme during the enzyme-catalysed reaction. The response current increases with increasing potential and temperature between 5 and 40 degrees C. The activation energy of the enzyme-catalysed reaction is 88.7 kJ mol(-1) The activity of the enzyme electrode is affected by pH. The optimum pH is 8.4. The result from the effect of pH on the maximum rate, i.e. maximum response current, indicates that the immobilized xanthine oxidase has two ionizing groups involved in the catalytic activity. The enzyme electrode has a fast response time and a high operational stability; its activity decreased by only 39% after 120 days. The response current increases linearly with increasing concentration of xanthine below 1.0 mmol dm(-3). This enzyme electrode can be used to determine the concentration of xanthine in human blood.