A new method for the amperometric detection of alkaline phosphatase activity based on hydroquinone recycling has been developed. p-Hydroxyphenyl phosphate is hydrolyzed by alkaline phosphatase to hydroquinone which, instead of being detected directly, enters an amplification cycle where it is oxidized to quinone at the electrode surface and then reduced back to hydroquinone by glucose oxidase in the presence of glucose. The consumption-regeneration cycle of hydroquinone results in an amplification factor of about 8. A detection limit of 2.5 x 10(-14) M of alkaline phosphatase was observed. Comparisons with the detection limits obtained with standard techniques (colorimetry, fluorimetry and chemiluminescence) were made under the same experimental conditions (same enzyme stock solutions, incubation time of 10 min and temperature of 37 degrees C).