Two hundred three testicular germ cell tumors were studied immunohistochemically for the presence of placental alkaline phosphatase (PLAP). Special emphasis was placed on the pattern and incidence of positive staining of intratubular malignant germ cells (ITMGCs) adjacent to tumors. 99% of cases with adjacent ITMGCs showed a positive staining reaction in some or all IT-MGCs present. Other germ cell elements showed at least a focal positive staining reaction in the following proportions: seminomas, 96%; embryonal carcinomas, 96%; yolk sac tumors, 25%; mature teratomas, 5%; immature teratomas, 4%; choriocarcinomas, 45%; and syncytiotrophoblasts, 43%. The staining pattern for seminomas tended to be diffuse, whereas for embryonal carcinomas the staining pattern was more focal. Yolk sac tumors stained inconsistently for PLAP and a positive reaction was limited to a small percentage of cells. Syncytiotrophoblasts, singly or in choriocarcinomas, also showed variable positivity. These results corroborate the fact that PLAP is a sensitive marker for ITMGC, seminoma, and embryonal carcinoma.