Over the past several years, the most widely used in vitro tests for mutagenicity have been those performed on bacteria. These include the Ames test, SOS, and SOS/umu chromotest[1,2]. The Ames test is the most commonly used system, but it is not automatable and requires several strains to detect different types of mutagens. Faster alternative tests include the SOS chromotest and the umu test, which are based on SOS promoter-linked Lac Z, which is activated by the SOS response to DNA damage in Escherichia coli and Salmonella typhimurium, respectively. The most important drawback of bacterial tests is that they are not capable of emulating the cell cycle controls or the repair mechanisms found in eukaryotic cells because the gene structure, gene regulation, and metabolism for prokaryotic cells are quite different from those of eukaryotic cells. In this way, the use of prokaryotic cells in mutagenicity screening for chemical substances has certain limitations.