Calibrating and using a new aerosol sizing instrument was described. The aerodynamic particle sizer (APS), the first commercial instrument to provide rapid, high resolution aerodynamic sizing of aerosols, was used by NIOSH for more than 2 years, either as a prototype or a commercial version. The APS functioned by accelerating an aerosol through a nozzle and passing it through a laser velocimeter. Eighty percent of the incoming aerosol was filtered and reintroduced as sheath air; the remaining 20 percent was retained for measurement. Aerosol particles were focused into the center of a nozzle and accelerated through the center of two laser beams. Particles in the nozzle lagged behind the air flow and the difference in velocity between the air and particles was related to the aerodynamic diameter of the particles. The APS was originally calibrated with a monodisperse aerosol of di-octyl- phthalate (DOP). Larger particles, 20 to 100 microns in diameter, were observed to flatten into oblate spheroids. The instrument was then calibrated with solid latex particles, and DOP and oleic-acid aerosols were studied to evaluate the effect of droplet flattening. Diameters of DOP and oleic-acid droplets were measured to be 20 percent smaller than the true aerodynamic diameter, for a diameter of 15 microns. The APS was tested with solid monodisperse particles of density 1.15 and 2.15. Shifting the density from 1.15 to 2.15 introduced an 8 percent increase in the measured aerodynamic diameter in the 8 to 14 micron range. When calculations were performed to simulate coincidence, the extent of coincidence observed did not agree with that calculated. The author concludes that the APS is a useful instrument for sizing particles having diameters of 0.8 to 20 microns. The distortion of oil droplets precludes their use as calibration material, unless one is measuring similar liquids.