Aurora Flight Sciences Corporation and partner, Draper Laboratory, propose to develop an on-orbit immuno-based label-free white blood cell counting system using MEMS technology (OILWBCS-MEMS) for human spaceflight experimental and medical monitoring practices. Our proposed system is designed to 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, and generating minimal bio-hazardous waste: SBIR topic X14.02 "On Orbit Cell Counting and Analysis Capability". The proposed detection technology leverages changes in optical transmission through a surface due to molecular binding (e.g., antibody-antigen binding). Antibodies specific to the white blood cell surface protein markers (antigens) are pre-coated on the sensor surface to recognize specific white blood cell types with inherently high specificity and sensitivity. In Phase I we developed surface chemistry and demonstrated surface chemistry sensitivity and specificity for total white blood cells and two lymphocyte subtypes (B-cells, CD4+ T-cells). During phase II we will develop a functional prototype of the OILWBCS-MEMS device to demonstrate that end-to-end operations from sample-in to signal-out produces clinically relevant results. The OILWBCS-MEMS design will include single-use replaceable cartridges for fluid loop and sensor components.