p63 is a novel transcription factor-encoding gene with sequence homology to p53. p63 proteins have epithelial stem-cell regulatory functions and play a critical role in tissue development. Study of p63 expression in testicular germ cell tumors has been limited. Thirty-four archival cases of testicular germ cell neoplasia were examined and stained with monoclonal anti-p63 antibody 4A4 using standard methods. Included were 19 seminomas, 1 pure teratoma, 3 pure embryonal carcinomas, 1 pure yolk sac tumor, and 10 mixed germ cell tumors. p63 staining was consistently positive in teratomas in areas of squamous differentiation and in basal reserve-like cells in foci of respiratory/endodermal differentiation. Strong p63 staining was observed within cytotrophoblasts of choriocarcinoma (1/1), whereas focal positivity was detected in embryonal carcinomas (4/10) and yolk sac tumors (2/5). Seminomas and intratubular germ cell neoplasia were p63-negative. These findings may suggest the presence of pluripotent p63-positive stem cell-like nests in yolk sac tumors and embryonal carcinomas or may represent areas of an occult teratoma phenotype undetectable histopathologically on hemotoxylin-eosin sections. p63 positivity in cytotrophoblasts of choriocarcinoma is consistent with gynecologic studies, possibly reflecting the role of p63 in the oncogenesis of neoplastic trophoblasts. The consistent p63 negativity in seminomas may reflect a precommitted embryonic precursor-like phenotype.