Production of chimaeras with embryonal carcinoma and embryonic stem cells enabled a very thorough investigation of the potency of these cells in the mouse. Human embryonal carcinoma and embryonic stem cell differ from their murine counterparts in a number of respects and, for obvious reasons, their potency is more difficult to assess. Recently, findings attesting to a surprising degree of plasticity of cells from adults have begun to emerge, which, aside from offering a possible further route to stem cell therapy, raise intriguing questions about the importance of lineage in the process of cellular diversification. Biomedical research is widely perceived to be advancing too fast to allow proper consideration of the implications of its clinical applications. Whilst this was clearly not true in the case of human in vitro fertilization, it has some validity regarding stem cell therapy, even though many of the issues are common to both. Casual use of the term 'embryo' proved unhelpful in the past debate on whether research on early stages of human development should be permitted. Likewise, introduction of the term 'therapeutic cloning' has complicated the present one regarding extension of such research to stem cell therapy.