Natural Killer cells are large granular cytotoxic lymphocytes that are critical to the innate immune system. Human NK cells express the surface marker CD56 and provide rapid responses to virally infected cells and tumor cells. Our CD56+ NK cells are isolated from peripheral blood of healthy donors using immunomagnetic micro-beads for positive selection. After isolation, the cells are immediately cryopreserved. The cells are characterized using flow cytometry and have a purity of >95%.