The construction and performance under flow-injection conditions of an integrated amperometric biosensor for hydrogen peroxide is reported. The design of the bioelectrode is based on a mercaptopropionic acid (MPA) self-assembled monolayer (SAM) modified gold disk electrode on which horseradish peroxidase (HRP, 24.3U) was immobilized by cross-linking with glutaraldehyde together with the mediator tetrathiafulvalene (TTF, 1mumol), which was entrapped in the three-dimensional aggregate formed. The amperometric biosensor allows the obtention of reproducible flow injection amperometric responses at an applied potential of 0.00V in 0.05molL(-1) phosphate buffer, pH 7.0 (flow rate: 1.40mLmin(-1), injection volume: 150muL), with a range of linearity for hydrogen peroxide within the 2.0x10(-7)-1.0x10(-4)molL(-1) concentration range (slope: (2.33+/-0.02)x10(-2)Amol(-1)L, r=0.999). A detection limit of 6.9x10(-8)molL(-1) was obtained together with a R.S.D. (n=50) of 2.7% for a hydrogen peroxide concentration level of 5.0x10(-5)molL(-1). The immobilization method showed a good reproducibility with a R.S.D. of 5.3% for five different electrodes. Moreover, the useful lifetime of one single biosensor was estimated in 13 days. The SAM-based biosensor was applied for the determination of hydrogen peroxide in rainwater and in a hair dye. The results obtained were validated by comparison with those obtained with a spectrophotometric reference method. In addition, the recovery of hydrogen peroxide in sterilised milk was tested.