Immunohistochemical localization of placental alkaline phosphatase (PlAP) has been performed on eighty‐two samples of normal (embryonic, fetal, infantile or adult), cryptorchid or tumorous testicular tissue. The isoenzyme could be demonstrated at the cell membrane of primitive, embryonic germ cells but not in other normal tissues. In‐situ carcinomas and seminomas were positively stained in 93% and 94% of cases respectively. The percentage of positivity decreased with tumour differentiation. It is suggested that PlAP is a marker of primitive germ cells and that it reappears in germ cell neoplasms by gene derepress