Human endometrial adenocarcinoma cell lines (KCC-1a and KCC-1b) were established from a resected endometrial carcinoma of a 67-year-old woman. The original tumor showed the bimorphic patterns of tubular adenocarcinoma and clear cell carcinoma. The cell lines have been maintained for 50 months through 100 passages. The doubling time of KCC-1a was 87 hours, whereas that of KCC-1b was 144 hours. The number of chromosomes of KCC-1a distributed in a range from 33 to 79, whereas that of KCC-1b distributed in a range from 64 to 82 (mode, 76). Functionally, both KCC-1a and KCC-1b cells showed the productions of estrone (E1) and estradiol (E2) but not estriol (E3) or progesterone (P). These cells grown in a serum-free medium secreted E1 (KCC-1a, 13.6 pg/ml; KCC-1b, 9.5 pg/ml) and E2 (10.5 pg/ml, 9.5 pg/ml) at passage 40. Neither KCC-1a nor KCC-1b cells contained estrogen or progesterone receptors. Tumor markers--alpha-fetoprotein, carcinoembryonic antigen, CA 125, and CA 19-9--were not detected. Both KCC-1a and KCC-1b cells produced tumors in nude mice xenografts. Histologically, the tumors of KCC-1a cells were clear cell carcinoma, whereas those of KCC-1b cells were tubular adenocarcinoma. These findings suggest that KCC-1a and KCC-1b cells will provide useful information to clarify the histogenesis of endometrial carcinoma combined with clear cell carcinoma.