A leukocyte filter for blood which includes a housing defining a blood inlet and outlet at respective ends thereof, with the housing enclosing and carrying a tubular, non-woven fiber filter element, with one of the inlet and outlet communicating with the interior of the tubular filter element, and the other of the inlet and outlet communicating with the exterior of the tubular filter element. In the tubular filter element, essentially all of the fibers define diameters of 10 to 100 microns, and have other diameter restrictions described herein, with the fibers being bonded together to define a generally fixed, average interstitial spacing which is less at the inner surface of the tubular filter element than at the outer surface thereof. The air permeability of the filter per square foot is preferably from essentially 15 to 45 SCFM at one half inch of water.