A microporous asymmetrical membrane formed of one or more layers wherein the "tight" side of the membrane has an "opened" face or otherwise highly-porous reticulated surface is described. The microporous asymmetrical membrane has high throughput and high flux, even when used for filtering viscous materials, such as serum or plasma. The membrane's surface can be formed by ablation or solvation, or in a two or more layered structure, through an appropriate selection of casting dopes.