Experimental evidence for the electrokinetic lifting of submicrometer colloidal particles in laminar flow through microcapillary tubes is presented. Electrokinetic lift is a new colloidal force originating when a charged particle and the diffuse part of the electrical double layer surrounding it are made to move relative to each other in the presence of another surface. This force occurs in the absence of an applied external field and is comparable to double-layer repulsion for low conductivity fluids. Our measurements of the average residence time of monodisperse polystyrene latexes being pumped under laminar flow through microcapillaries have shown a strong flow rate dependence using eluants of less than 10(-3) M ionic strength. This behavior was most noticeable at the lowest electrolyte concentration (3 x 10(-6) M) using a surfactant-free eluant. Deviation from theoretically-predicted separation factors (a parameter equal to the ratio of the mean displacement velocities of particle and eluant) became more pronounced with increasing particle size and decreasing tube diameter. The mathematical model which was developed from first principles predicts that particles in this size range will not be influenced significantly by hydrodynamic forces, thus precluding any dependence of the average particle velocity on the eluant flow rate. A repulsive force of electrokinetic origin may be responsible for the anomalous data which show good qualitative agreement with theoretical results when we incorporate a recently derived lift force expression into the model.