Urea-sensitive sensors, based on immobilized urease and the differential antimony electrode, have been constructed. The calibration curve of the sensor is linear up to 1·6–2·0 mmol litre −1 in the stationary or kinetic mode of operation. The sensors retain 50% of their sensitivity for 16–17 days. The analyser for urea, constructed on the basis of the sensor, ensures quick and precise urea determination in pure blood.