The cell line HTOA was established from a well-differentiated human ovarian serous cystadenocarcinoma. This line grew well and without interruption for 51 months and was subcultivated over 130 times. The cells were epithelial in shape, and neoplastic and pleomorphic features, a jigsaw puzzle-like arrangement, desmosomal junctional complexes, and multilayering without contact inhibition. The chromosome number was stable at a hypertetraploid range. The culture cells transplanted into BALB/c nude mice and or hamster cheek pouch produced serous cystadenocarcinomas. The cells were found to produce an antigen (CA125) of ovarian cancer, both in vitro and in vivo. The CA125 levels correlated with cellular proliferation in vitro and also with tumor growth, in the nude mouse. These results indicate that the amount of CA125 in the serum is a good marker for detecting early stages of ovarian cancer and in particular for the evaluation of anticancer drugs.