A neural origin of Ewing’s sarcoma (ES) has often been suggested and we have demonstrated neurofilament protein expression in ES cells. However, only the 200-kD subunit has been revealed in all of the ES cells analyzed. The 160- and 68-kD subunits were always absent. For these reasons, we have attempted to induce neural differentiation in 3 ES cell lines with different types of inducers: tetradecanoylphorbol-13-acetate (TPA) retinoic acid and nerve growth factor. When the cell lines were cultured for 7 days with TPA (10^-9M) or retinoic acid (10^-7M), only the 68-kD neurofilament subunit was slightly induced. No inducation was obtained when nerve growth factor was used, even at a 21-day culture. These results are in agreement with the putative neural origin of ES and may indicate an abnormal expression of neurofilament proteins in this tumor.