Ki-1 (CD30) antigen expression has been considered to be restricted to hematopoietic tissues including the recently described anaplastic large cell lymphoma and Reed-Sternberg (RS) cells in Hodgkin's disease. Its presence on some activated lymphocytes in non-neoplastic lymphoid tissues has been used as evidence that such cells might represent the physiologic counterpart of RS cells. In this study expression of CD30 antigen in 117 nonhematopoietic tumors was investigated using monoclonal antibody Ber-H2. The antigen was regularly expressed in frozen section (strongly) and paraffin section (less strongly) by embryonal carcinomas (8 of 10 studied) and the embryonal elements of mixed germ cell tumors (4 of 4), but not in other types of germ cell tumors (0 of 11) or nonhematopoietic tumors (0 of 92). Normal adult, neonatal, and fetal testes were negative for CD30 antigen, as were other fetal tissues and placenta. Ki-1 antibody gives similar results in frozen section. These findings have implications for theories suggesting an origin of RS cells from activated lymphocytes. They are also important for determining the diagnostic significance of CD30 positivity in a tumor of unknown origin, and suggest possible new uses for CD30 antibodies in routine diagnostic immunohistology.