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We report on a fabrication and packaging process for a microsystem consisting of a mass-based protein detector and a fully integrated preconcentrator. Preconcentration of protein is achieved by means of a nanofluidic concentrator (NC), which takes advantage of fast nonlinear electroosmotic flow near a nanochannel-microchannel junction to concentrate charged molecules inside a volume of fluid on the order of 1 pL. Detection of preconcentrated protein samples is accomplished by passing them through a suspended microchannel resonator (SMR), which is a hollow resonant cantilever serially connected to the NC on the same device. The transit of a preconcentrated sample produces a transient shift in the cantilever's resonance frequency that is proportional to the density of the sample and, hence, the concentration of protein contained in it. A device containing both NC and SMR structures was produced using a novel fabrication process which simultaneously satisfies the separate packaging requirements of the two structures. The initial testing of this prototype device has demonstrated that the integrated SMR can accurately measure the concentration of a bovine serum albumin solution, that was preconcentrated using the integrated NC. Future improvements in the fabrication process will allow site-specific surface modification of the device and compatibility with separation methods, which will create opportunities for its application to immunoassays and universal detection.