MODEL RELEASED. Stem cell blood analysis. Technician holding a sample of blood stem cells that has been prepared for examination in a flow cytometer. The cells have been tagged with a fluorescent antibody to a surface protein (CD 34) found on some blood stem cells. The cells are passed through a beam of light in the cytometer and any fluorescing cells are counted. The stem cells, which are able to develop into any type of blood cell, were harvested from placental and umbilical cord blood. This technique is known as a CD 34+ enumeration assay and determines the concentration of stem cells in a sample.