An enzymatic biosensor for hydrogen peroxide based on a horseradish peroxidase (HRP)-ferrocene carbon paste modified electrode and coated with a layer of electrochemically generated poly(o-aminophenol) is reported. A linear calibration curve is obtained over the range 1 x 10(-8) M to 1 x 10(-5) M. The biosensor responds to hydrogen peroxide ina few seconds and has a detection limit of 8.5 x 10(-9) M. The response of the biosensor is diffusion controlled at low substrate concentrations. Flow injection assays of hydrogen peroxide at a sampling rate of 150 injections per hour with a relative standard deviation of 0.8% (50 samples) are possible. Applicability of the sensor for measurement of hydrogen peroxide in real samples (milk) was demonstrated.