An enzyme electrode for L-lactate measurements in various biological media was prepared with an immobilized bacterial respiratory chain fixed to a Clark electrode. The enzymatic film, which was easy to prepare, contained bacteria immobilized in gelatin, tanned with glutaraldehyde. This electrode was sensitive to 0.1 mM L-lactate and could be utilized to 10 mM. The apparent K50 was 5 mM. Less than 8% of the respiration rate with L-lactate was measured with D-lactate and succinate. The competitive inhibitors D-lactate and pyruvate had a K50 of 50 mM. They could be quantitatively measured by inhibition in a range between 5 and 50 mM. It was also possible to discriminate between L-lactate and various metabolites of the respiratory chain: L-malate, succinate, 3-glycero-phosphate or NAD(P)H. Growing E. coli on 1% D-L-lactate as the sole carbon source in minimal medium induced L-lactate respiration tenfold. All other respiratory activities remained below 10% of the activity with L-lactate. A computerized instrument allowed successive measurements every 3 min for more than 10 h with the same enzymatic film. Most of the measured samples required dilution but no clarification or purification. This enzyme electrode may have many applications in basic research (metabolism, enzymology) and applied research (blood, yogurt, juices, wine).