In this Bureau of Mines study, a filter with a 20- by 20-inch-square cross section was inserted into the exhaust ducting of a 4-stroke- cycle, 90-hp diesel engine, and the efficiency of removal of total particulate was measured gravimetrically as a function of time. Removal efficiencies were close to 90 pct during the main portion of several extended runs employing several combinations of filter beds. Variations in engine load had little effect on collection efficiency. Particle size as measured by a Bureau-developed online instrument centered on 0.07 Um based on number count. Removal efficiency increased rather sharply for both larger and smaller particles.