A thermostable lactate oxidase (from Aerococcus viridans) based bioreactor was elaborated for L-lactate determination in dairy products. The enzyme was immobilised in a thin-layer enzyme cell connected into a flow injection system with an amperometric detector. The linear measuring range was 0.1–1 mmol/L L-lactate (R2=0.99; linear regression equation: y=81.2x+3.3). The detection limit was 0.05 mmol/L. 40 Samples were analyzed per hour. Considering reproducibility: the signals measured were very stable for over 800 injections, after this period the activity of the cell decreased slowly. The method was used for the control of dairy products and for monitoring milk freshness.