The function of the engine air cleaning filter is to remove the particulate matter in the intake air to protect the engine and its components from wear and contamination. For a specific filter, the efficiency is a function of the size of the particles being collected and the air flow velocity through the filter. Traditional tests of engine air cleaners are based on the use of specific test dusts, such as the AC Coarse and AC Fine, to determine the mass collection efficiency. However, they do not provide information on the size dependent performance of the filters, and the variation in filter performance under different particle challenge conditions. The use of a fractional efficiency test method will help to provide this missing information.The purpose of this paper is to describe a fractional efficiency test system that has been designed to evaluate the fractional cleaning efficiency of engine air cleaning filters in the size range between 0.3 and 10 mm particle diameter. Test particles can be NaCl, generated by atomization, or dry dust particles, aerosolized by a powder disperser. The filter is challenged by these particles, and the efficiency is determined by counting and sizing the particles upstream and downstream of the filter with a laser particle counter. The mass collection efficiency is estimated from the fractional efficiency results. The performance of this system and the important factors in the design are discussed. Results for typical cellulosic filter for engine air cleaning applications are reported.