Sarcosine oxidase has been immobilized in the polyaniline film by the electrochemical doping method. The response current of the polyaniline sarcosine electrode is a function of the applied potential, and increases with increasing pH in the region 7.0 to 8.75 and with increasing ionic strength of the buffer. The optimum pH of the immobilized sarcosine oxidase is 8.75. The optimum temperature is 39.6 degrees C. The activation energy of the enzyme-catalyzed reaction is 32.0 kJ mol(-1). The apparent Michaelis-Menten constant K-m’ is 1.7 mM. The enzyme electrode has a high operational stability. The response current of the enzyme electrode increases linearly with increasing concentration of sarcosine in the range below 1.0 mM, and is not affected by formaldehyde at concentrations less than 0.01 mM. Thus, the polyaniline sarcosine oxidase electrode can be used to determine sarcosine concentration.