Abstract: A testing apparatus is disclosed which is useful for determining if air filters are providing clean/sterile air within acceptable aseptic processing and/or clean room operation acceptable contaminant levels. The advantage of the disclosed testing apparatus is that air filters may be tested "on-line" during a production process as opposed to the traditional testing methods which included removal of the air filter for off-line testing. Less down-time is experienced using the disclosed testing system. A series of valves positioned at the outputs from the air filters, which allow communication with a particle counter, are controlled by a computer which sequentially activates the valves to test the contamination level of the air which passed through an air filter during the production process.