the authors describe a new biosensor for monitoring the concentration of enzyme glucose, lactate, and other substrates that are metabolized by an oxidation process. the method is based on the finding that enzymes having fad as a prosthetic group change their fluorescence during interaction with a substrate. typical enzymes that have been studied include glucose oxidase (god), lactate mono-oxygenase (lmo), and cholesterol oxidase (chod). their fluorescence is monitored via fiber optic light guides at wavelengths above 500 nm, following fluorescence excitation at around 410-450 nm. the relative fluorescence intensities of the enzymes vary to a large extent, being highest for lmo, and rather low for chod. a characteristic feature of this sensor is the narrow dynamic range which usually does not exceed 3 mm. this can be explained in terms of enzyme kinetics and diffusional processes. the sensor has the advantages of full reversibility and analytical wavelengths that are compatible with plastic or glass fiber optics it is fairly simple in construction because the enzyme acts as both the recognition and transduction element. the method also has been applied successfully in an flow injection analysis-like type of arrangement