Cell therapies require that stem cells be “purged” of contaminating tumor cells or more immature progenitor cells. Although methods using viruses or antibody selections have been developed for this purpose, they tend to be expensive and labor intensive. On page 882, Eppich et al. show that hematopoietic stem cells can be selected by simply applying pulsed electric fields that create pores in cellular membranes, eventually resulting in cell death. The sensitivity of cells to lysis is proportional to their size; this permits the extremely small hematopoietic stem cells to be rapidly enriched. The authors found that in blood cell samples contaminated by tumor cells and mature blood cells, the method could enrich for stem cells by at least two orders of magnitude.