This paper presents the development and evaluation of a high-volume, multiple rectangular (slit) geometry jet impactor. Operating with a preselective inlet that removes particles larger than 2.5 mum in aerodynamic size, the impactor has been designed to separate Ultrafine (< 0.15 mum) from the accumulation mode range (0.15 < d(p) < 2.5 mum). Particles are accelerated by passing through 10 parallel slits nozzles, each 12.5 cm long by 0.0125 cm wide. The average jet velocity in each rectangular jet is approximately 5800 cm s(-1). Following acceleration, particles larger than approximately 0.15 mum impact on quartz fiber strips, each 12.5 x 0.5 cm(2) while the aerosol fraction smaller than 0.15 mum penetrates through the outlet of the impactor. The impactor operates at a flow rate 550 1 min(-1) and at a very low pressure drop of 0.020 kPa. The performance of the multi-slit impactor was validated in laboratory and the field tests. Laboratory experiments conducted with monodisperse PSL particles as well as polydisperse ammonium nitrate, sulfate and indoor aerosols corroborated the 50% cutpoint of the impactor at 0.15 mum. Field test comparisons between the high-volume multi-slit impactor and the Microorifice Uniform Deposit Impactor (MOUDI) showed that the accumulation and Ultrafine mode concentrations of particulate nitrate, sulfate, elemental and organic carbon are in very good agreement (within 10% or less). This impactor has been developed primarily as a separator of Ultrafine from accumulation mode particles for use in human exposure studies to concentrated ambient ultrafine aerosols. High-volume collection of size-fractionated particulate matter accomplished by this impactor further enables investigators in the field of environmental health to conduct toxicological studies using ambient accumulation and Ultrafine mode particles in vitro as well as by means of intratracheal instillation. (C) 2002 Elsevier Science Ltd. All rights reserved.