Abstract: The pore size distribution of a microporous separation medium, such as a membrane or gel, is modified so as to provide it with a sharp upper cut-off of preselected molecular size. The modification is effected by first filling the pores of the separation medium with a volatile liquid, and then evaporating a controlled amount of the volatile liquid to form voids at the entrances to the pores. A concentrated solution of a cross-linkable or polymerizable pore-blocking agent, such as a protein, enzyme or polymeric material, is then applied to the surface of the separation medium. The pore-blocking agent is a material which is insoluble in the volatile liquid and whose molecular size distribution has a lower limit corresponding to the preselected molecular size, whereby the pore-blocking agent selectively enters only those pores larger than the preselected size and remains in the voids at the entrances to the pores.