The mixed mesodermal tumor has an admixture of carcinoma and sarcoma. The HIRS-BM tumor cell line, derived from the mixed mesodermal tumor, was transplanted into the peritoneal cavity of BALB/c nude mice, and the metastatic foci were examined histologically and karyologically. The HIRS-BM cells produced mixed mesodermal tumors (admixture of adenocarcinoma and rhabdomyosarcoma) resembling the original tumor. The normal interstitial tissue was not present on the boundary of the sarcoma and carcinoma. These results support the combination theory as the cause of mixed mesodermal tumor.