It has proved to be impossible to culture epithelial cells from the gastrointestinal tract of adult animals. Researchers have had to use either cell lines derived from new-born rat small intestine or colon carcinoma cell lines that have retained some of the properties of the gastrointestinal mucosa. We have described a method for establishing conditionally immortalized cell lines from the small intestine, colon and liver from tissue obtained from a transgenic mouse carrying a temperature sensitive mutant of the SV40 large T gene (the "Immortomouse"). This immortalizing gene has proved to be useful for establishing cell lines from a number of transgenic mice following cross-breeding of the "Immortomose" with the transgenic mouse of interest. These cell lines are being used in numerous studies. In this review we describe the methods for developing such lines and list the range of cell lines that have been developed from colon, small intestine, stomach, liver and pancreas.