Researchers have tried for at least 20 years to develop a normal human colonic cell line suitable for in vitro studies of human colonic diseases. We report a breakthrough development of two normal colon-derived cell lines. They are designated NCM356 and NCM425. The cells were collected from the histologically normal colonic margin of patients undergoing resection for colon adenocarcinomas and grown in culture. Since NCM356 and NCM425 have now been subcultured 22 and 19 times, each has undergone more than 40 population doublings. Neither cell line has shown evidence of terminal differentiation. Immunohistochemical characterization studies demonstrated that they are epithelial cells. They variably expressed subsets of other markers, including tumor markers, but did not grow in soft agar. NCM356 did not form tumors, whereas NCM425 was tumorigenic in immunodeficient mice. These two cell lines represent the first successful in vitro culture of human colonocytes derived from normal mucosa. NCM356 is closer to normal, but seems to represent an early stage of cell transformation, possibly correlated with immortalization. In contrast, in vitro culture of the NCM425 cell line appears to have selected for later progression to malignancy. These lines are important resources for studying colon cancer and the physiology of intestinal cells.