Biochemically functionalized magnetic beads are being used as labels in the development of rugged, handheld bioassays. These magnetic labels can be adapted for use in both protein and DNA assays. A palm-sized magnetic excitation module has been fabricated and demonstrated with integrated Giant Magnetoresistive (GMR) detector arrays. The arrays have 20 sites, each of which is a GMR magnetic sensor underneath a 200 micron diameter functionalizable assay surface. The sites are functionalized by attaching a reference protein or oligonucleotide to the surface. The assay is typically a sandwich-type assay, though other techniques are possible. The sensitivity of the assay is quite good, with reports of better than 1 femtoMolar detection limits. The integrated detectors can detect a single immobilized magnetic label, and has a dynamic range of better than 10^3. The ultimate performance of a given assay is dependent on microfluidics and sample handling. The magnetic detection scheme has several advantages over common optically based techniques. The excitation device and detector array are both solid state devices and are inherently rugged. Both are amenable to low cost mass- manufacturing techniques. And it is possible, with sophisticated sample handling, that PCR amplification may be unnecessary due the ability to detect a single label. Magnetic forces may also be used to enhance reaction rates and improve specificity. This presentation will address issues related to fabrication and surface modification of the GMR sensor array chip. The most common assay surfaces include silicon nitride, silicon dioxide, aluminum dioxide, and gold. In all cases, the surface quality must be very high to achieve predictable and desirable assay results. With continued development, this magnetic bioassay platform should prove to be ideal for applications where high speed multiplexed sandwich-type assays are done on a disposable platform.