detection of chemicals and toxins in multi-component mixtures has previously required a knowledge of the identity of the species to be detected and expensive, time consuming analytical procedures. by focusing on the physiological targets (receptors, antibodies, and enzymes), which these agents attack, it is possible to have detection of classes of compounds without these restraints. this is achieved by coupling an optically tagged recognition site (e.g., receptor) to the surface of a fiber optic waveguide (fcwg) and taking advantage of the interaction between the optical tag and the evanescent wave effect of the fowg. the binding of a toxin to the receptor alters the fluorescence of the tag, hence the amount of light conveyed by fowg to a detector. in this report, we describe the critical features of a receptor-based biosensor: immobilization of receptor proteins; coupling of fluorescent tags to the receptor; development of a rugged, replaceable sample chamber assembly; and the design of a hardened optical system