Current techniques in high-speed cell sorting are limited by the inherent coupling among three competing parameters of performance: throughput, purity, and rare cell recovery. Microfluidics provides an alternate strategy to decouple these parameters through the use of arrayed devices that operate in parallel. To efficiently isolate rare cells from complex mixtures, an electrokinetic sorting methodology was developed that exploits dielectrophoresis (DEP) in microfluidic channels. In this approach, the dielectrophoretic amplitude response of rare target cells is modulated by labeling cells with particles that differ in polarization response. Cell mixtures were interrogated in the DEP-activated cell sorter in a continuous-flow manner, wherein the electric fields were engineered to achieve efficient separation between the dielectrophoretically labeled and unlabeled cells. To demonstrate the efficiency of marker-specific cell separation, DEP-activated cell sorting (DACS) was applied for affinity-based enrichment of rare bacteria expressing a specific surface marker from an excess of nontarget bacteria that do not express this marker. Rare target cells were enriched by >200-fold in a single round of sorting at a single-channel throughput of 10,000 cells per second. DACS offers the potential for automated, surface marker-specific cell sorting in a disposable format that is capable of simultaneously achieving high throughput, purity, and rare cell recovery.cell sorting ͉ microfluidics C ell sorters are capable of separating a heterogeneous suspension of particles into purified fractions and thus have become an indispensable tool in biology and medicine. Emerging applications of cell sorting technology span a broad spectrum of pharmaceutical and biomedical fields that range from cancer diagnostics to cell-based therapies (1-3). The most widely used methodologies for cell separation are magnetic-activated cell sorting (MACS) and fluorescence-activated cell sorting (FACS). MACS is a selection technique that is capable of capturing a large number of target cells in parallel (4); however, the purity and recovery in MACS typically have large variances (5). In contrast, FACS relies upon serially screening each cell, yielding high performance in cell recovery and purity (6). However, because of the serial nature of its operation, FACS allows for a comparatively low throughput, typically in the range between 10 4 and 10 5 cells per second (7). Regardless of the mechanism, the performance of cell separation is typically characterized by three metrics. ''Throughput'' gauges how many cell characterization and sorting operations can be executed per unit of time, ''purity'' is the fraction of the target cells in the collection vessel, and ''recovery'' is the fraction of the input target cells successfully sorted into the collection vessel. Demands placed on cell sorting technologies continue to increase, because cell sorting applications are expanding and biological questions are becoming more complex (7). For example, rare cell sort...