A new type of fiber-optic biosensor based on bacterial cytoplasmic membranes (CPM) as the biological recognition element and an oxygen sensitive dye layer as the transducer is described for the detection of lactate. CPMs from bacteria with an induced lactate oxidase system are adsorbed onto a cellulose disk. The disk is fixed mechanically over an oxygen sensitive siloxane layer on the distal end of an optical fiber. This system detects lactate with no interference from glucose, fructose or glutamic acid.