Abstract. With conventional capillary electrophoresis (CE), it was difficult to directly analyze samples containing proteins as a result of the irreversible adsorption of proteins onto the inner surface of the capillary column. This difficulty, however, was completely overcome by adding N-dodecylphosphocholine (DPC, a phosphobetaine-type zwitterionic surfactant) to the background electrolyte (BGE). DPC made two essential contributions to the determination of common inorganic cations in the protein-containing samples: protein adsorption onto the capillary walls was completely avoided, and the resolution of the analyte cations was essentially improved. The optimal BGE for analysis of biological samples was found to be 5mM DPC, 5mM copper(II) acetate/10mM ethylenediamine (pH8). The detection limits (signal-to-noise ratio=3 and UV at 215nm) of sodium, potassium, calcium, magnesium, and ammonium ions were 25, 31, 24, 45, and 60M, respectively. These five species of the common inorganic cations in human saliva samples were detected successfully within 2min by the proposed system with direct sample injection.