The establishment of human embryonic stem (ES) cell lines has brought great potential and expectations for regenerative medicine and pharmaceutical research, because many types of human cells could be produced by their unlimited growth and differentiation in culture. Primate and human ES cell lines have been established from blastocysts of monkey and surplus human blastocysts from fertility clinics. They showed several differences compared to mouse ES cells, including a tendency to produce the trophectoderm lineage and a different expression pattern of surface antigens. This may reflect species-specific differences, or these primate ES cells could represent earlier stages of development than mouse ES cells. Also, they show no response to the LIF and gp130 signals, which are widely used to repress spontaneous differentiation of mouse ES cell colonies. We have established several ES cell lines from blastocysts of the cynomolgus monkey. They can be maintained in culture as stem cell colonies, and they produce several differentiated cell types in culture. When such ES cells were transplanted into SCID mice, they produced teratomas containing many differentiated tissues.