Twenty-one cases of seminoma (including testicular seminoma, ovarian dysgerminoma and extragonadal germinoma) were reviewed for the cell types responsible for the production of alpha-fetoprotein (AFP) and human chorionic gonadotropin (HCG). Histologically the cases included seventeen classical seminomas and 4 anaplastic seminomas. The latter had some mononuclear and multinuclear giant cells. All 4 patients with anaplastic seminoma had elevated levels of serum AFP, and each of these cases contained AFP producing tumor cells identified by immunoperoxidase staining. All seminomas of patients with elevated serum levels of HCG were of the classical type but HCG producing tumor cells could not be identified by immunoperoxidase staining. Immunoreactivity to anti-AFP was found in some large mononuclear cells and anaplastic cells. To explain these results, we propose that the large mononuclear cell is a multipotential cell capable of differentiating into a germ cell, yolk sac and embryo, and that the anaplastic seminoma cells might represent a stage on the continuum of cellular differentiation from the large mononuclear cells to germ cells. The multinuclear giant cell does not appear to be essential for the production of either AFP or HCG in seminoma.