A biosensor for the specific determination of L-ascorbic acid in fruit juices and vitamin C tablets was developed using ascorbate oxidase (EC 126.96.36.199) from cucumber (Cucumis sativus L.) in combination with a dissolved oxygen probe. Ascorbate oxidase immobilized with gelatin using glutaraldehyde and fixed on pretreated teflon membrane served as an enzyme electrode. The phosphate buffer (50 mM, pH 7.5) and 35 degrees C were established as providing the optimum conditions. The biosensor response depends linearly on L-ascorbic acid concentration between 5.0 x 10(-5) and 1.2 x 10(-3) M with a response time 45 s. The biosensor is stable for more than 2 months, while more than 200 assays were performed. The results obtained for fruit juices and tablets were compared with DCIP (2,6 dichlorophenolindophenol) method. (C) 1999 Elsevier Science B.V. All rights reserved.