The enzyme urease and paramagnetic Fe3O4 particles were incorporated into microcapsules (3-4 m in size) formed using sodium polystyrene sulfonate and polyallylamine hydrochloride polyelectrolytes. Microcapsules were attracted to the gate of a pH-sensitive field effect transistor by a permanent magnetic field to form a bioreceptor on its surface. The immobilization technique used required no chemical reagents to activate and regenerate transistor's surface. The volume of the measured sample was 3 I; the total reaction volume, 30 mu l. Biosensor signals developed over a time of the order of 30-150 s. The biosensor enabled the detection of urea within the concentration range of 0.03 up to 100 mM. The enzyme activity and signal values of the biosensor did not decrease within 30 days. Measurements of real blood samples yielded a satisfactory correlation of biosensor data and clinical data (correlation coefficient, 0.78). (C) 2015 Elsevier Ltd. All rights reserved.