Direct electron transfer of immobilized horseradishperoxidase on gold colloid and its application as a biosensorwere investigated by using electrochemicalmethods. The Au colloids were associated with a cysteaminemonolayer on the gold electrode surface. A pairof redox peaks attributed to the direct redox reaction ofhorseradish peroxidase (HRP) were observed at theHRP/Au colloid/cysteamine-modi®ed electrode in 0.1 Mphosphate buffer (pH 7.0). The surface coverage of HRPimmobilized on Au colloid was about 7.6×10-10 mol/cm2.The sensor displayed an excellent electrocatalytic responseto the reduction of H2O2 without the aid of anelectron mediator. The calibration range of H2O2 was 1.4mMto 9.2mMwith good linear relation from 1.4mMto 2.8mM. A detection limit of 0.58 mM was estimated at asignal-to-noise ratio of 3. The sensor showed good reproducibilityfor the determination of H2O2. The variationcoef®cients were 3.1 and 3.9% (n＝10) at 46 mM and 2.8mM H2O2, respectively. The response showed a Michaelis－Menten behavior at higher H2O2 concentrations. TheKMapp value for the H2O2 sensor was found to be 2.3 mM.