Specially prepared microporous Neosepta ion-exchange membranes were investigated to establish a correlation between their structural characteristics (pore-size distribution, porosity) and permeability to components of immunoglobulin (Ig) fractions of mouse ascitic fluids. The solutions to be separated contained IgG(1) with specificity to horseradish peroxidase or to the heavy chain of human IgM, some other proteins, and a large amount of ammonium sulfate (0.22-0.35 M). Analysis of the membrane morphology carried out by scanning electron microscopy and mercury porosimetry showed that the membranes possess a polymodal pore-size distribution. There are large open pores (400-600 and 200-300 nm in diameter) on the membrane surfaces, but the void volume of the membranes is a system of connected pores of smaller diameters (from 60-100 to 7-10 nm). The main part of the pores in the membranes displaying the best separation ability was 8-17 nm in diameter. It was found that highly porous charged membranes (relative porosity 58-60%) with low ion-exchange capacity (0.02-0.1 meq/g) made it possible to achieve the desired desalination degree of protein mixture (80-83%) within 5-7 h instead of 5 days needed in the traditional dialysis. Moreover, the amount of separated accompanying proteins reached 25-30% depending on membrane porosity and the quality of specific IgG(1) was considerably improved.