In this study, we employed the microfluidic ratchet mechanism to perform perpetual microfiltration of leukocytes from whole blood. Building on results from cell deformability studies performed using AFM32, micropipette aspiration33, 34 and its microfluidic derivative35, we hypothesize that erythrocytes, leukocytes and leukocyte subpopulations can be sensitively separated based on their distinct cell deformability. Previously, the microfluidic ratchet mechanism has been shown to be able to effectively separate viable circulating tumor cells from whole blood36 with significant improvements in yield over conventional methods as well as to separate erythrocytes infected with Plasmodium falciparum 37 from uninfected erythrocytes to improve the sensitivity of malaria diagnosis. Using oscillating flow of cells through an array of funnel shaped microstructures, we show that whole blood can be processed without clogging or fouling the filter matrix. Consequently, this process enables highly selective separation of leukocytes and erythrocytes, as well as demonstrates the potential for phenotypic sorting of leukocytes.