A human tumor cell line MCAS, which was derived from mucinous cystadenocarcinoma of the ovary, was established. The tissue specimen was obtained on December 1, 1983 and the culture was maintained in Ham's F-12 medium supplemented with 20% fetal calf serum. Continuous growth made possible cell transfer every 3-5 days and the seventy-third passage was achieved on August 15, 1984. This cell line has the following characteristics. The doubling time of the MCAS cell line was 27 hours and the saturation density was 5.1 X 10(4)/cm2. The cultured cells appeared monolayer and the cellular arrangement was a pavement-like pattern. Rather large cells with vacuoles in the cytoplasm appeared in the confluent state and PAS staining proved to be positive specifically in these large cells. A chromosomal analysis revealed a peritetraploid tendency. By heterotransplantation of MCAS cells to the nude athymic mouse, the tumor developed and had both cystic and solid components. The cystic content was mucinous. Histopathological diagnosis of the tumor was papillary adenocarcinoma and alcian blue staining was positive. The characteristics of the original mucinous cystadenocarcinoma have been precisely maintained in this cell line.