We present the design of a microchannel with dynamic geometry that imparts different flow rates to different cells based on their physical properties. This dynamic microchannel is formed between a textured surface and a flexible membrane. As cells flow through the microchannel, the height of the channel oscillates causing periodic entrapment of the larger cells, and as a result, attenuating their velocity relative to the bulk liquid. The smaller cells are not slowed by the moving microstructure, and move synchronously with the bulk liquid. The ability of the dynamic microchannel to selectively attenuate the flow rate of eukaryotic cells is similar to a size-exclusion chromatography column, but with the opposite behavior. The speed of smaller substances is attenuated relative to the larger substances in traditional size-exclusion chromatography columns, whereas the speed of the larger substances that is attenuated in the dynamic microchannel. We verified this property by tracking the flow of single cells through the dynamic microchannel. L1210 mouse lymphoma cells (MLCs), peripheral blood mononuclear cells (PBMCs), and red blood cells (RBCs) were used as model cells. We showed that the flow rate of MLC is slowed by more than 50% compared to PBMCs and RBCs. We characterized the operation of the microchannel by measuring the velocity of each of the three cell types as a function of the pressures used to oscillate the membrane position, as well as the duty cycle of the oscillation. This journal is © The Royal Society of Chemistry 2011