A process is provided for rapidly and non-destructively evaluating the particle retention characteristics of porous membranes using a novel liquid-porosimetric technique. A ratio of two membrane permeabilities is measured at preselected operating conditions using a pair of mutually immiscible fluids, one of which is employed as a membrane wetting agent and the other used as an intrusion fluid. The first operating condition, a particular transmembrane pressure, is chosen so as to selectively intrude those pores accessible to a given sized particle, such as a virus. The second operating condition, either a second preselected transmembrane pressure or a water permeability measurement, is chosen such that nearly all of the membrane pores are intruded. The ratio of the two permeabilities obtained with each set of operating conditions is thus the percentage of total flow through the membrane pores accessible to a given sized particle. By experimentally measuring this permeability ratio on membranes with known particle retention behavior, a calibration curve relating the permeability ratio to the particle retention characteristics can be constructed. From this standard correlation, the particle retention capabilities of an unchallenged membrane can be predicted in a non-destructive manner solely by measuring its permeability ratio.