A human mediastinal tumor in a 19-year-old male containing the elements of embryonal carcinoma, yolk sac tumor, choriocarcinoma and teratoma, was successfully transplanted into nude mice. The transplanted tumor was transferred to tissue culture and a cell line, designated as HMT cells, was established. The morphology of HMT cells showed various cell types; mainly flat epithelial cells, piled-up colonies of cells, polygonal cells and teratoid elements. The chromosomes of HMT cells were heteroploid and were distributed from 48 to 66 on the 30th day in culture. The HMT cells produced AFP in cultured medium and had characteristics of embryonal carcinoma cell in high-level alkaline phosphatase. The histology of the tumor obtained by retransplantation of HMT cells into nude mice was mainly that of embryonal carcinoma with few teratoid elements. In long term culture HMT cells changed to less-differentiated cells.
