a cross flow microfilter comprises a filter housing having a flow chamber wherein a fluid is introduced on one side and a concentrate is removed on the opposite side a collection chamber from which a filtrate is removed , and at least one microporous membrane over whose one surface the fluid to be filtered flows in a parallel stream and whose other surface is oriented toward the collection chamber , the membrane having pores of a width between about 0 . 1 micron and 30 micron , the improvement wherein the pores of the membrane have slit - shaped cross - sections and given profiles , and are formed by a plurality of webs arranged in rows with predetermined spacing and by terminating plates which cover the webs , and the webs are applied onto one of the two plates by x - ray lithographic means , x - ray depth lithographic - galvanoplastic means , or molding and galvanoplastic means .