Protein biomarkers provide the key diagnostic information for the detection of disease, risk of disease progression, and a patient's likely response to drug therapy. Potential biomarkers exist in biofluids, such as serum, urine, and cerebrospinal fluid. Unfortunately, discovering and validating protein biomarkers are hindered by the presence of high-molecular-weight proteins, such as serum albumin and immunoglobulins, which comprise 90% of the proteins present in these samples. High-abundance, high-molecular-weight proteins mask the low-molecular-weight (LMW) proteins and peptides using conventional protein detection methods. Candidate biomarkers are believed to exist in very low concentrations and comprise less than 1% of serum proteins, and may be highly labile as well. Therefore, it is imperative to isolate and enrich LMW proteins from complex mixtures for biomarker discovery. This chapter describes a continuous -elution electrophoresis method, based on molecular weight sieving, to isolate specific molecular weight fractions for mass spectrometric, western blotting, or protein array analysis.