In April 2005, the U.S. National Academy of Sciences (NAS) published its Guidelines for Human Embryonic Stem Cell Research. These voluntary guidelines are among the most permissive in the world—in a country that prohibits federal funding of research to derive human embryonic stem (hES) cells (cells that can self-renew or differentiate into most cells in the human body). One of the few research prohibitions in the NAS guidelines concerns the creation of certain kinds of human–nonhuman chimeras. A chimera is an organism with a mixture of cells from two different organisms, from the same or different species. Figure 1 provides a useful overview of different types of chimeras.This work was supported, in part, by a Stem Cell Network grant to Françoise Baylis and Jason Scott Robert and a CIHR grant to Françoise Baylis. We sincerely thank Alan Fine, Rich Campbell, Cynthia Cohen, and Tim Krahn for helpful comments on an earlier draft of this paper. Thanks are also owed to Tim Krahn for his research assistance. An earlier version of this paper was presented to the Department of Bioethics and the Novel Tech Ethics research team (www.noveltechethics.ca). We thank the participants at each of these meetings for their helpful comments.