To produce urease-based bioselective elements, the solutions containing the enzyme (5%) and BSA (5%) were prepared. The proteins were dissolved in 5 mM phosphate buffer, pH 6.5, with glycerol (10%). The mixture for reference membrane was pre pared in the same way but no enzyme, only BSA was taken, the final concentration of which was 10%. The solutions obtained were deposited onto the transducers using micropipettes «Eppendorf (the total volume of 0.1-2.5 jul); to cover entirely the transducer working surface the volume of each membrane was about 0.1 ul. Each membrane contained equal total amounts of protein. Biosen- sors were incubated for 30 min in an exsiccator with saturated