The primary tumour tissues from 13 teratomas were investigated, 3 cases of immature teratoma (1 of pure type and 2 of mixed type), 5 cases of dermoid cyst, and 5 cases of mature solid teratoma, with special reference to N-myc gene amplification, productivity of neuron specific enolase (NSE), and the presence of double minutes (DMs). The possible relationship between these variables and malignancy of the tumours was also examined. N-myc gene amplification and NSE production were recognized in the primary tumour tissues and the first and the third passage cultured cells of all of the 3 cases of immature teratoma containing immature neural tissues. In 2 cases DMs were recognized. In dermoid cysts and mature teratoma, neither N-myc gene amplification nor NSE production were recognized in either the primary tumour tissues or cultured cells. The chromosomes were normal. Malignancy of teratoma is generally decided by the clinical stage and histological grade, but a more securely based decision is necessary where an immature teratoma contains immature neural tissues. The presence of N-myc gene amplification, NSE productivity and the presence of DMs may be valuable.