Recent progress in stem cell research provide promising ways to isolate stem cells with varying proliferative and differentiation abilities. Embryonic stem cells (ESCs) are considered to be the most primitive cells and they exhibit infinite potential to self-renew and proliferate. ESCs can differentiate into cells of all three germ layers, whereas most adult stem cells (ASCs) are tissue specific and have shown limited proliferative and differentiation potential. ASCs often exhibit genetic changes depending upon the age of the tissue or individual from which they were isolated. Among the ASCs, the umbilical cord blood stem cells (CBSCs) are more primitive and exhibit lower levels of immunogenicity. Studies have suggested that cancer stem cells (CSCs) are resistant to radiation and escape during chemotherapy treatments and cause the recurrence of cancer. We hypothesized that stem cells (SCs) with varying renewal and differentiation potential may be resistant to radiation to variable degrees. To test this hypothesis, we investigated the effect of gamma radiation on the mouse ESCs (mESCs) and two human ASC types isolated from cord tissue (CTSCs) and cord blood (CBSCs). These SCs varied in potency and exhibited varying degree of resistance to gamma radiation. mESCs and CBSCs were more resistant to lower doses than CTSCs. However, CTSCs survived exposure to the highest dose, although the survival frequency was low. Gamma irradiation did not show an apparent change in the morphology, survivability, or differentiation potential. Studies are in progress to determine molecular changes and the differentiation potential of irradiated cells as compared to non-irradiated cells. These studies may be useful for transplantation during or after chemotherapy in oncology patients.