This study presents the development and bench-testing of a versatile aerosol concentration enrichment system (VACES) capable of simultaneously concentrating ambient particles of the coarse, fine and ultrafine size fractions for conducting in vivo and in vitro studies. The VACES consists of three parallel sampling lines (concentrators), each operating at an intake flow rate of 110 l min(-1). Coarse particles are concentrated using a single round nozzle virtual impactor. Concentration enrichment Of PM2.5 and ultrafine particles is accomplished by first drawing air samples through two parallel lines, having 2.5 and 0.18 mum cutpoint pre-impactors, respectively, to remove particles larger than these sizes from the air sample. Both of the smaller PM fractions are drawn through a saturation-condensation system that grows particles to 2-3 mum droplets, which are subsequently concentrated by virtual impaction. A diffusion dryer is used in the fine and ultrafine concentrators to remove excess vapor and return the concentrated particles to their original size, prior to supplying them for in vivo exposures. The VACES can also provide highly concentrated liquid suspensions of particles of these three modes for in vitro toxicity studies. This is accomplished by connecting the concentrated output (minor) flows of each of the VACES parallel concentrators to a liquid impinger (BioSampler), used in a modified configuration, to collect particles under near-ambient pressure. Detailed laboratory characterization of the individual components of the VACES are presented in this paper, including evaluation of its ability to preserve particle mass, number, and chemical species during the concentration enrichment process. Our experimental results showed that concentration enrichment is accomplished with very high efficiency, minimal particle losses and without any significant dependence on particle size or chemical composition.