Blood is a valuable tissue containing cellular populations rich in information regarding the immediate immune and inflammatory status of the body. Blood leukocytes or white blood cells (WBCs) provide an ideal sample to monitor systemic changes and understand molecular signaling mechanisms in disease processes. Blood samples need to be processed to deplete contaminating erythrocytes or red blood cells (RBCs) and sorted into different WBC sub-populations prior to analysis. This is typically accomplished using immuno-affinity protocols which result in undesirable activation. An alternative is size based sorting which by itself is unsuitable for WBCs sorting due to size overlap between different sub-populations. To overcome this limitation, we investigated the possibility of using controlled osmotic exposure to deplete and/or create a differential size increase between WBC populations. Using a new microfluidic cell docking platform, the response of RBCs and WBCs to deionized (DI) water was evaluated. Time lapse microscopy confirms depletion of RBCs within 15 s and creation of > 3 μm size difference between lymphocytes, monocytes and granulocytes. A flow through microfluidic device was also used to expose different WBCs to DI water for 30, 60 and 90 s to quantify cell loss and activation. Results confirm preservation of ~100% of monocytes, granulocytes and loss of ~30% of lymphocytes (mostly CD3+/CD4+) with minimal activation. These results indicate feasibility of this approach for monocyte, granulocyte and lymphocyte (sub-populations) isolation based on size.