We describe a method of using heat to produce any pattern of differential permeability in commercially available fluid permeable membranes. An aluminum heating block is machined with specific patterns, heated, and placed in contact with a continuous sheet of polypropylene membrane in order to create impermeable regions in the otherwise permeable membrane. The permeability of the membrane to fluid can be altered by this method in any pattern chosen. Such differentially permeable membranes can be used in high-density, multi-well filtration plates for chemical syntheses or biological assays. (C) 2003 Elsevier Science B.V. All rights reserved.