The various histologic components of 39 germ cell tumors of the testis were studied with indirect labeled immunoperoxidase technique for the presence of the following tumor-associated antigens: alpha-fetoprotein (AFP), alpha 1-antitrypsin (A1AT), chorionic gonadotropin (HCG), specific pregnancy beta 1-glycoprotein (SP1), human placental lactogen (HPL), carcinoembryonic antigen (CEA), and ferritin (Fe). Embryonal carcinoma components were positive for AFP in 9/16, for A1AT in 6/16, for CEA in 2/16, and for Fe in 14/16. All yolk-sac tumor components were positive for AFP and Fe, and 8/13 contained A1AT and 2/13 CEA. Epithelium in teratoid components was positive for AFP in 5/14 cases, for A1AT in 8/14, for CEA in 7/14, and for Fe in 8/14. Syncytial trophoblast in choriocarcinoma components and syncytial giant cells were all positive for HCG, SP1 and HPL. In addition, tumor giant cells in two nonseminomatous tumors and in one seminoma contained HCG. Otherwise, all pure seminomas were negative for all antigens, except for Fe, which was positive in 12/16 cases. Demonstration of this functional aspect of germ cell tumors of the testis may clarify problems of classification and elucidate histogenesis. Furthermore, immunohistochemical demonstration of tumor-associated antigens is of value in the management of the patient as an indicator of which tumor markers should be used for monitoring the treatment. In addition, the presence of tumor-associated antigens may be used in prognostic evaluation.