Over the past few years, technological procedures have been developed for utilizing stem cells in transplantation medicine. Human embryonic stem (ES) cells can produce an unlimited number of differentiated cells and are, therefore, considered a potential source of cellular material for use in transplantation medicine. However, serious clinical problems can arise when uncontrolled cell proliferation occurs following transplantation. To avoid these potential problems, we genetically engineered human ES cell lines to express the herpes simplex virus thymidine kinase (HSV-tk) gene. Expression of the HSV-tk protein renders the ES cells sensitive to the U.S. Food and Drug Administration-approved drug ganciclovir, inducing destruction of HSV-tk+ cells at ganciclovir concentrations that are nonlethal to other cell types. The reversion rate of engineered cells was low even under prolonged selection with ganciclovir. The HSV-tk+ clones retained a normal karyotype and the ability to differentiate to cells from all three germ layers. Most importantly, tumors that arose in mice following subcutaneous injection of HSV-tk+ human ES cells could be ablated in vivo by administration of ganciclovir. By utilizing these cell lines, safety levels can be improved in transplantations involving tissues derived from human ES cells.