Forced penetration of large hydrosoluble polymer chains through track-etched membranes has been investigated as a function of both solvent flow rate in the pores and the ratio of polymer hydrodynamic radius to pore radius. We measure the rejection coefficient R obs from retentate and permeate mean concentrations, and its corrected value R including polymer accumulation at the membrane. The variations of R as a function of solvent flow rate per pore in adimensional units collapse into the same curve well fitted by de Gennes' ''suction model''. This curve, universal for flexible polymers in good solvents, leads to an estimate of the critical penetration flow.