There is a need for the ability to perform rapid and specific bio-monitoring of a wide range of toxins for use by clinicians, environmental epidemiologists, and emergency medical responders. Ultra-miniature GMR magnetic sensors based on very mature technology developed by the magnetic recording industry are well- suited to form the basis of an innovative toxin assay system. While the proposed approach can easily incorporate the parallel detection of multiple targets, a single toxin, Bisphenol A (BPA), will be specified in this feasibility demonstration. Our multidisciplinary team will generate custom aptamers tailored to uniquely bind with BPA using the SELEX procedure. These aptamers will use BPA molecules to connect functionalized magnetic nanoparticles (MNPs) to the GMR sensor surface for detection. The sensor will be integrated with a microfluidic network within a disposable cassette to facilitate controlled analyte preparation, introduction, and testing. Custom electronics will be developed to read out the GMR sensor and detect the presence of BPA. The use of advanced technologies enables the extreme miniaturization of each individual component, allowing the entire assay system to be used in the field at the point of care. The proposed system combines the use of functionalized aptamers, magnetic nanoparticles, ultra-miniature magnetic sensors and microfluidic system design techniques in a highly innovative solution to the problem of bio-monitoring and the detection of a large family of chemical compounds in a patient's environment.