We determined whether a carbohydrate antigen in serum, CA19-9, serves as a marker for embryonal carcinoma. Serum CA19-9 was serially measured in 18 male patients with a germ cell tumor. Five of 8 patients with an embryonal carcinoma had an elevated serum CA19-9. The elevated serum CA19-9 returned to normal in parallel to the clinical response to the treatment. Patients with a seminoma had a normal serum CA19-9 irrespective of their tumor size. Paraffin-embedded specimens of germ cell tumors from 9 of the 18 patients and 10 other patients were stained with the monoclonal antibody against CA19-9. These immunohistochemical stainings showed that embryonal carcinomas and mature teratomas were consistently positive and seminomas were often faintly positive for CA19-9, whereas choriocarcinomas, yolk sac tumors and immature teratomas were negative for CA19-9. These results indicate that serum CA19-9 may be a useful serum marker for embryonal carcinomas.