The most basic objection to human embryonic stem (ES) cell research is rooted in the fact that ES cell derivation deprives embryos of any further potential to develop into a complete human being^,. ES cell lines are conventionally isolated from the inner cell mass of blastocysts^,, and, in a few instances, from cleavage stage embryos^,,,. So far, there have been no reports in the literature of stem cell lines derived using an approach that does not require embryo destruction. Here we report an alternative method of establishing ES cell lines—using a technique of single-cell embryo biopsy similar to that used in pre-implantation genetic diagnosis of genetic defects—that does not interfere with the developmental potential of embryos. Five putative ES and seven trophoblast stem (TS) cell lines were produced from single blastomeres, which maintained normal karyotype and markers of pluripotency or TS cells for up to more than 50 passages. The ES cells differentiated into derivatives of all three germ layers in vitro and in teratomas, and showed germ line transmission. Single-blastomere-biopsied embryos developed to term without a reduction in their developmental capacity. The ability to generate human ES cells without the destruction of ex utero embryos would reduce or eliminate the ethical concerns of many.