We present the first microfluidic method for extracting proteins from heterogeneous fluids by precipitation. The new method comprises an automated protocol for precipitation of proteins onto surfaces, rinsing the precipitates to remove impurities, and resolubilization in buffer for further analysis. The method is compatible with proteins representing a range of different physicochemical properties, as well as with complex mixtures such as fetal bovine serum and cell lysate. In all cases, the quantitative performance (measured using a fluorescent assay for % recovery) was comparable to that of conventional techniques, which are manual and require more time. Thus, this work represents an important first step in efforts to develop fully automated microfluidic methods for proteomic analyses.