In this work, multi-wall carbon-nanotube (MWCT) is evaluated as transducer, stabilizer and immobilization matrix for the construction of amper-ometric biosensor based on lactate dehydrogenase (LDH) and Meldola's Blue (MB). The amperometric response was based on the electrocatalytical properties of MB to oxidize NADH, which was generated in the enzymatic reaction of lactate with NAD(+) under catalysis of LDH. It is shown that the employed materials are promising as electrochemical mediators and enzyme stabilizers. The enzyme was immobilized onto the MWCT adsorbed with MB by cross-linking with glutaraldehyde. The dependence on the biosensor response for lactate was investigated in terms of pH, supporting electrolyte, LDH and NAD(+) amounts and applied potential. The amperometric response for lactate using this biosensor showed excellent sensitivity (3.46 mu A cm(-2) mmol L-1), operational stability (around 96.5% of the activity was maintained after 300 determinations) and wide linear response range (0.10-10 mmol L-1). These favorable characteristics allowed its application for direct measurements of lactate in blood samples. Good agreement was found between the results obtained by the developed biosensor and other well established analytical method, indicating that it should be highly selective for the lactate. Moreover, the biosensor also presented excellent stability in operational conditions. (c) 2007 Elsevier B.V. All rights reserved.1241269276