Neuron-specific enolase (NSE) is a well-known marker of tumours that have neuroendocrine origin. High levels of NSE have also been described in various types of testicular germ cell neoplasms, particularly in seminomas. To evaluate the presence of NSE in testicular carcinoma-in situ (CIS), a preinvasive stage of testicular germ cell tumours, a panel of CIS tissue specimens was examined. Fifteen of 18 (83 per cent) CIS samples showed immunohistochemical staining with anti-NSE monoclonal antibody. Immunoreactivity has also been found in overt testicular germ cell tumours, including seminomas, non-seminomas, and a mixed germ cell tumour. As the co-existence of high NSE production and gene amplification of N-myc has been reported in some tumours, including germ cell tumours, the expression of the protein product of N-myc was also examined in this study, but only sporadic cases showed N-myc staining. These results are evidence against a relationship between NSE and N-myc in testicular germ cell tumours. The high expression of NSE in CIS and overt germ cell tumours may be due to the increased gene dosage effect associated with the overrepresentation of isochromosome 12p.