The filtration and pressure drop-flow rate characteristics of MC 1478 filter paper was determined over wide ranges in pressure 1/20 to 1/5 atm.) ard temperature (--40, 0, and 70 deg F). The retention of SiO/sub 2/ spherical particles by the filter paper at a flow rate of 5000 ft/min at 1/20 atm. pressure and 70 deg F was found to be essentially 100%. The retention (generally 30 to 75%) for aluminum oxide particles (about 40 A) and pressure drop were determined over wide ranges in pressure, temperature, and flow rate, The application of kinetic theory to small particle diffusion in a gas is discussed. The theory of diffusion of particles from a streaming gas to the tube wall was evolved and applied to the estimation of particle size. (C.J.G.)