Tumor cells derived from a metastatic mediastinal lymph node of a thirty six year old man with large cell lung cancer were transplanted into the subcapsule of the kidney of a nude mouse. The growing tumor was transplanted into a nude mouse twice and, thereafter, cultured in vitro. After the twelfth passage in vitro, the cell (NUTIY) was transplanted into subcutaneous tissue of a nude mouse and formed a rapidly growing tumor, showing the histological pattern similar to the large cell lung cancer of the patient. However, the chromosomal analysis revealed only murine chromosomes for NUTIY and any transfected human genes were not found in the genes of NUTIY by means of Alu probe. H-ras oncogene of NUTIY was amplified 4 folds, but that of the primary lung cancer and the metastatic lymph node of the patient was on a level with normal cells. NUTIY might be a cancer cell which a murine cell had been transformed into by some carcinogen in vitro.