Porcine and bovine cell lines derived from the inner cell mass (ICM) or epiblasts of blastocysts have been maintained over extended periods of time and characterized by morphology, identification of some stem cell markers and, in few cases, by production of chimaeric offspring. However, germ line transmission in chimaeras has never been obtained. Due to this incomplete characterization of the cell lines, the expression embryonic stem (ES)-like cells is presently used in pig and cattle. The ICM or epiblast can be isolated from the blastocyst by whole blastocyst culture, mechanical isolation, or immunosurgery, and they are generally cultured on feeder cells. The resulting ES-like cells may be differentiated in vivo by chimaera and teratoma formation or in vitro by embryoid body formation and monolayer induction. It is likely that more well characterized and stable porcine and bovine ES cell lines will be available over the coming years. However, in order to reach this goal further systematic research is needed. Such cell lines hold promises for developing adequate models for human ES cell therapy and they may open for new avenues for the production of genetically modified animals as the ES cells ahve indefinite selfrenewal capacity and may be more prone for homologous recombination. Finally, ES cell lines established from blastocysts, which are genetically modified to carry human disease genes, may be differentiated into particular cell populations that may allow for studies of disease mechanisms at the cellular level in vitro as a parallel to studies on whole animal models born from transfer of similar genetically modified embryos.