Although glucose sensors with millimolar sensitivity are still the norm, there is now a developing interest in glucose sensors with micromolar sensitivity for applications in minimally invasive sampling techniques such as fast microdialysis and extraction of interstitial fluid by iontophoresis and laser poration. In this regard, the glucose binding protein (GBP) with a binding constant for glucose in the micromolar range is of particular relevance. GBP is one of the soluble binding proteins found in the periplasmic space of Gram-negative bacteria. Because of its hinge-like tertiary structure, glucose binding induces a large conformational change, which can be used for glucose sensing by attaching a polarity sensitive fluorescent probe to a site on the protein that is allosterically responsive to glucose binding. Correspondingly, the resulting optical biosensor has micrornolar sensitivity to glucose. Because binding is reversible, the biosensor is reusable and can be stored at 4 degrees C for 6 months without losing its sensitivity. In this paper, we show the feasibility of using the GBP biosensor to monitor glucose in microdialysis. The effect of perfusion rate, bulk glucose concentration and temperature on microdialysis efficiency was determined. Additionally, the glucose concentrations in mammalian cell culture were monitored to demonstrate the applicability of this sensor in complex and dynamic processes over a period of time. As the sensor is sensitive to micromolar glucose, high dialysis efficiency is not required when the bulk glucose concentration is within the millimolar physiological range. Thus, a perfusion rate of 10 mu L/min or faster can be used, resulting in delay times of 1 min or less.