Indirect immunoperoxidase staining was carried out on human testicular tumors using monospecific antibodies against placental (Regan) and intestinal isoenzymes of alkaline phosphatase (ALPase). The very high incidence of seminoma (approximately 90%) revealed positive staining of placental ALPase mainly on the cell membrane of tumor cells, whereas none of the seminoma showed presence of intestinal isoenzyme. Placental isoenzyme was not recognized in any embryonal carcinoma and interstitial cell tumor. The epithelial cells of the glandular elements of teratoma occasionally exhibited strong staining for intestinal ALPase and weak staining for placental ALPase. The appearance of Regan isoenzyme in seminomas might be considered possible conversion of hepatic to placental isoenzyme, a consequence of malignant transformation of spermatogenic cells. Regan isoenzyme appears to be a new tumor marker for seminoma and the frequent identification of Regan isoenzyme in seminoma may disclose a unique biologic characteristic of this germinal tumor.