Indirect immunofluorescence and radioimmuno-assay with specific rabbit antisera demonstrated the occurrence of alphafetoprotein (AFP), carcino-embryonic antigen (CEA) and placental alkaline phosphatase (PLAP) in primary testicular tumor cells. Embryonal carcinomas had AFP- and CEA-containing cells, sometimes PLAP. PLAP and sometimes CEA were found in seminoma cells. Sera from patients with advanced non-seminomatous tumors could contain any of these antigens or any combination of them. Sera from patients with seminomas had raised PLAP or CEA. PLAP appears to be a new marker for seminoma.