One of the major issues in stem cell biology resides in the development of efficient methods for differentiating stem cells into specific cell types, which will serve as ideal sources for cell therapy, drug screening, and so forth. In the case of neuronal differentiation of embryonic stem (ES) cells, it has been previously shown that SDIA (stromal cell-derived inducing activity) produced by PA6 stromal cells can promote neuronal differentiation of both mouse and primate ES cells, in particular into dopaminergic neurons (Kawasaki et al., 2000; Kawasaki et al., 2002b), which bears a potential clinical value of treating Parkinson`s Disease (PD). In this study, we have attempted to co-culture human ES cells (SNU-hES3) at various different stages, such as undifferentiated, embryoid body and neurosphere, with PA6 stromal cells. The initial results of this experiment indicate that human ES cells can be induced to differentiate into neuronal lineages by the co-culture with PA6 stromal cells, although the differentiation efficiency was influenced by the stages of human ES cells that were used in the co-culture experiments. In order to increase the efficiency of neuronal differentiation, the culture conditions obviously need to be further optimized. Nevertheless, these results will provide an important framework for the neuronal differentiation of human ES cells.