We analyzed the sensitivity of a polyclonal antibody to placental alkaline phosphatase (PLAP) in labelling testicular germ-gell neoplasms, by utilizing the peroxidase-antiperoxidase technique. The immunoreactivity of 89 germ-cell tumors for PLAP was as follows: 98% of cases with seminomatous elements were PLAP-positive; 97% of embryonal carcinomas and 85% of endodermal sinus tumors also showed reactivity. Cytotrophoblastic cells were focally immunoreactive in one of two cases with choriocarcinomatous elements. Staining of PLAP was strongest and most diffuse in seminomas. Intratubular germ-cell neoplasia (ITGCN) or carcinoma in situ was present in 53 (84%) of 63 specimens that had adjacent seminiferous tubules available for evaluation; PLAP was demonstrated in 98% of these. In addition, the germ-cell elements in 11 gonadoblastomas were immunoreactive for PLAP. One of 17 cases of undescended testes had ITGCN that was also strongly immunoreactive, but the remaining 16 cases were negative. Five dysgenetic gonads without ITGCN were studied, and one was immunoreactive for PLAP. Three testicular biopsy specimens from infertile men without ITGCN were PLAP-negative. Our findings indicate that PLAP is a highly sensitive marker for seminomas, for the majority of embryonal carcinomas and endodermal sinus tumors of the testis, and for ITGCN.