A new human endometrial adenocarcinoma cell line, Ishikawa cells, was established from an endometrial adenocarcinoma from a 39-year-old woman and has been maintained in vitro for more than 3 years. The cells were found to form a monolayer in a mosaic fashion and to tend to pile up. Population doubling time was calculated to be about 36, 29 and 27 hours at the 9th, 40th and 50th generations, respectively. The modal chromosomal number of the cells fell in a diploid range. Histology of the tumor induced in athymic nude mice showed it to be a well differentiated adenocarcinoma which closely resembled the original human tumor. Estrogen receptor and progesterone receptor were demonstrated to occur not only in the induced tumor in athymic nude mice but also in in vitro culture cells. From the fact that the cell growth was maintained in an estrogen-free medium, it appeared that the cells had no estrogen dependency.