A new mechanism for separation in gel permeation chromatography (CPC) or gel filtration has been developed based upon the postulate that flow occurs through the gel phase. On the basis of the hydrodynamic nature of the separation mechanism, the new nameHydrodynamic Fractionationis proposed. A theory has been formulated for the case of equal pore size in the gel phase and equal sized spaces between the beads. This theory predicts the elution volume vs molecular weight curve. The flow-rate dependence of the separation peaks is investigated.