If all individual cell types of the body could be clonally isolated and stocked, similar to cDNA or genomic DNA libraries, they would be invaluable for studying the tissue and cellular functions. We developed a new method of establishing conditionally immortalized cell lines that retain differentiated cell functions similar to the original tissues, using temperature-sensitive (ts) simian virus 40 large tumor antigen gene transgenic animals. In this review the properties of such conditionally immortalized cell lines and their possible applications are discussed.