N-myc gene amplification of the gynecological malignant tumor cell lines and a neuroblastoma cell line was studied by the Southern hybridization method along with the production of neuron-specific enolase (NSE) by these cell lines. N-myc amplification and NSE production were observed side by side in three cell lines: neuroblastoma cell line HSNB, endodermal sinus tumor cell line HAEST, and malignant teratoma cell line HUOT. However, N-myc amplification and NSE production disappeared gradually following successive passages of the HAEST and HUOT lines. With respect to the HUOT line, these parameters disappeared along with the cells of nervous origin. N-myc amplification and NSE production were not observed in nine other cell lines.