A cell line, designated RMG-I, has been established from ascites of a patient with ovarian clear cell carcinoma. RMG-I cells have grown well for more than 6 years with mirror ball formation. Chromosome analysis revealed aneuploidy with a modal number of 47 and 3 marker chromosomes. The histological characteristics of the transplanted tumor were similar to those observed in the original tumor which was composed of dark cells, clear cells, and hobnail cells. Thus, the RMG-I cell line was identified to be an ovarian clear cell carcinoma. Immunohistochemically, basic fetoprotein (BFP) and ferritin were demonstrated in the original tumor, the cultured cells, and the transplanted tumor. Furthermore, in order to clarify whether these 3 kinds of cells originate from one stem cell or not, monoclonal cell population were transplanted into nude mice, and its histological investigation revealed that 3 types of cells existed in the transplanted tumor, proving that they originate from one stem cell.