This paper presents a theoretical and experimental exploration of the prospects for performing separations on size-or shape-polydispersed solutions of macromolecules or colloidal particles via multistage countercurrent contact between liquid and porous phases, Fractional extraction is shown to yield an arbitrarily sharp cut with sufficiently many stages. It is therefore possible in principle to sharpen a particle size distribution about any prescribed size (which can be varied by adjusting the liquid flow rate without changing the porous phase), and to achieve molecular shape-selective separations. Experiments are reported that demonstrate the relatively rapid attainment of partitioning equilibrium for colloidal polystyrene particles within a single-stage fixed-bed contactor packed with controlled-pore glass. A comprehensive convection-diffusion model explains a decaying temporal oscillation in the particle concentration arising in the experiments. Implications of the results for the large-scale processing of macromolecular and particulate dispersions are discussed. It is noted that the scale-up potential of the present technique is not restricted to separations by size-exclusion, and applies equally well in principle to affinity-based and other separations.