A laboratory method that measures the number of cells, the percentage of live cells, and certain characteristics of cells, such as size and shape, in a sample of blood, bone marrow, or other tissue. The presence of tumor markers, such as antigens, on the surface of the cells are also measured. The cells are stained with a light-sensitive dye, placed in a fluid, and then passed one at a time through a beam of light. The measurements are based on how the stained cells react to the beam of light. Flow cytometry is used in basic research and to help diagnose and manage certain diseases, including cancer.