A test method employing a Continuous Aerosol Monitoring (CAM) system was developed to determine air filter performance. Overall and fractional filter efficiency and pressure drop can all be obtained as a function of time, dust loading, or dust particle size. The on-line CAM system simultaneously analyzes the concentration and size distribution of the dust upstream and downstream of the test filter. A MicroVAX computer is used to analyze the data. The overall filter efficiency determined by the CAM system is comparable to that obtained using the gravimetric method.