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The conception and implementation of biosensors for monitoring of sea andriver water quality are nowadays important topics in environmental research, and the monitoring of water for the presence of toxic substances, even in trace quantities, is foreseen to reach widespread use in the 90s. Because of their high sensitivity and ability to recognize a wider range of substances, immunosensors are particularly suited to this type of measurement. Optical immunosensors based on the TIRF (Total Internal Reflection Fluorescence) technique couple the selectivity of the antigen-antibody reaction to the spatial selectivity of the evanescent wave at a refractive boundary. The configuration for the TIRF sensor we have developed is based on a quartz optical fiber with a high numerical aperture. The influence of selected physico-chemical parameters that characterize the environment in which the sensor must operate, such as, ionic strength, pH, and temperature, has been evaluated. The experimental results have been compared with theoretical models of the optical response of TIRF sensors. In addition the concept of a competitive immunosensor system for non-fluorescent analytes is briefly analysed and the use of sensor reconditioning by photoactivation, by means of photochromic compounds, is suggested to improve the sensor time response.