ABSTRACT .... The fractional efficiencies of nine industrial dust collectors, including three cartridge filters, four baghouse filters, and two electrostatic precipitators, have been determined. The dust collectors were challenged in the laboratory by a calcium carbonate powder of 4. 7 ~m mass median diameter with a geometric standard deviation of 1.9 at various concentration levels. A recently developed novel sarnpling and measuring system involving the use of a real-time optical particle counter was used to mea= sure the upstream and downstream dust concentrations of the collectors. The performance of these collectors has been compared using a "figure of merit" quafity factor. Under laboratory test conditions, the cartridge filters gave the best performance, with collection efficiency exceeding 99.999%. The downstream mass concentrations for all the units tested were below the NIOSH/OSHA limits of 10 mg/m³ for total mass and 5 mg/m³ for respirable mass. This indicates that all units tested meet the necessary codes for operating at recirculating mode under ideal laboratory conditions° Field tests of similar dust collectors were performed. The field test results showed considerably more variation due to the difference in operating parameters, dust types, and maintenance. This suggests that the use of a secondary filter and/or installing a downstream dust rnonitor is a prudent measure in recirculation applications.