On the basis of analysis of literature data the determination of critical particle-to-pore size ratio F-cr = r(s)/R(h) (where r(s) is the minimal radius of a protein molecule which is retained by the membrane with a mean hydraulic pore of radius R(h)) for ultrafiltration membranes with asymmetric structure was carried out. It was shown that F-cr for these membranes has a value of 0.3 +/- 0.2 which coincides with the previous estimations of this ratio for isotropic ultrafiltration membranes.