Potentiometric creatinine microsensors were fabricated by use of a composite film consisting of polyion complex (PIG), which contains creatinine iminohydrolase (CIH), and the electrochemically inactive polypyrrole (iPPy). The PPy/PIC-CIH composite film electrode displayed potential response to creatinine concentration owing to a pH change occurring during the enzymatic reaction. The sensitivity was varied by using different polyions of the PIG. With poly-L-lysine and sebacic acid, the iPPy/PIC-CIH electrode demonstrated a high sensitivity (29.8 mV decade(-1)) and a low detection limit (1 x 10(-6) mol dm(-3)) which allow one to monitor creatinine concentration in the physiological range.