novel microporous , asymmetric , integral , composite piolyfluorocarbon , e . g ., polytetrafluoroethylene , membranes are disclosed . these membranes are made up of two or more sheets of microporous fluorocarbon polymer having different average pore sizes , arranged to provide an increasing gradations in average pore size from one surface of the membrane to the other , and are useful as microfilters for removing fine particle contaminants , including bacteria , from fluids such as those encountered in the electronics and pharmaceutical industries .