Aurora Flight Sciences Corporation and our partner, Draper Laboratory, propose to develop an on orbit immuno-based, label-free, white blood cell counting system for simultaneous counting of peripheral blood cell subpopulations, including total white blood cells, the five white blood cell differential subgroups, and various lymphocyte subtypes, such as CD4 and CD8 positive cells, using Draper's MicroElectroMechanical Sensor (MEMS) based microfabricated arrayable Adhesive Stress Electrostatic Sensor (ASES) technology. The proposed ASES sensor uses a capacitance read-out method to electronically measure the sensor membrane displacement due to the surface stress caused by molecular binding, (e.g., antibody-antigen binding). Antibodies specific to the white blood cell surface protein markers (antigens) are precoated on the ASES sensor membrane to recognize the specific white blood cell types with inherently high specificity and sensitivity. Our proposed cell counting system can meet NASA's requirements for a microgravity compatible, miniaturized, light weight peripheral blood cell counting instrument capable of on-orbit cell counting, without high energy lasers, requiring minimal sample volume or exogenous (sheath) fluid to operate, and generating minimal biohazardous waste. This ASES blood cell counting system, once developed, can stand alone for white blood cell differential and subtype count, or become a complimentary instrument to others available on-orbit.