Human embryonic stem (ES) cells are undifferentiated pluripotent cells derived from the inner cell mass of blastocyst stage embryos. These unique cell lines have the potential to form virtually any cell type in the body and can be propagated in vitro indefinitely in an undifferentiated state. These cells are capable of forming embryoid bodies (EB) that contain cells from all three embryonic lingages. Here, we have characterized the neurogenic effects of retinoic acid (RA, 10-6 M) on human ES cells (Royan H1) in vitro. For morphological and imunocytochemical evaluation, antibodies against microtubules-associated protein 2 (MAP-2), neurofilament protein, neuron-specific tubulin-III and neuron-specific enolase (NSE) were used. The results showed that more than 70% of embryoid bodies differentiated into neuronal cells as compared to a 10% change in the control group and the cells expressed neuron specific molecules (MAP-2, Neurofilament protein, tubulin III and NSE). These findings show that human ES cells have great potential to become as an unlimited cell source for neurons in culture. These cells may then be used in transplantation therapies for neural pathologies.