We immunohistochemically studied TCL1 protein expression in different histologic types of 63 testicular germ cell tumors: 23 seminomas, 14 embryonal carcinomas, 4 teratomas, 2 yolk sac tumors, and 20 mixed germ cell tumors. The 20 mixed germ cell tumors contained components of seminoma (n = 10), embryonal carcinoma (n = 18), teratoma (n = 16), yolk sac tumor (n = 7), and choriocarcinoma (n = 3). We also examined 40 cases of intratubular germ cell neoplasia, unclassified type (IGCNU). Positive immunoreactivity for TCL1 was observed in 91% of the seminoma samples (30/33) and all IGCNU cases. In contrast, no TCL1 expression was detected among the nonseminomatous germ cell tumors. In the context of testicular germ cell neoplasia, the presence of TCL1 protein appears restricted to IGCNU and seminoma, suggesting association with an undifferentiated state and loss of protein expression with tumor differentiation. Immunohistochemical evaluation of TCL1 expression may have usefulness in substantiating a diagnosis of IGCNU or seminoma and in the separation of seminoma from nonseminomatous germ cell tumors.