Respirable and nonrespirable particles are separately collected from the atmosphere in a size selective sampler consisting of two Nuclepore filters in series. The first filter has pore diameters of 12 micrometers and collects, to a close approximation, the nonrespirable fraction as defined by the ACGIH criteria. The second filter has pore diameters of 0.2 micrometer and collects the respirable fraction. Experimental measurements of the efficiency of the 12 micrometer filter are found to be in good agreement with theoretical predictions based on calculated particle trajectories in a flow field that satisfies the Navier-Stokes equations. Atmospheric samples were collected using both the techniques of series filtration and of virtual impaction. A mass and elemental analysis of the aerosol deposits using gravimetric and X-ray fluorescence techniques demonstrated that the results from the two types of sampler are comparable. Filter loading as great as 90 micrograms/sq cm was investigated and found to produce no change in filter performance as measured by the location of the 50% cutpoint.