It is theoretically impossible to extraporate the toxicity of drugs in humans from data obtained with experimental animals. In fact, accumulated data on species differences in the drug metabolism can provide only a little information for the estimation of the metabolism and the related toxicity of drugs under development in humans. The recent development in the molecular biology has realized the quantitative estimation of genetic distance between humans and experimental animals. Thus, this study was undertaken to estimate the distance between humans and experimental animals to propose the use of monkeys in an initial phase of the development of a new drug. Cell lines carrying human drug metabolizing enzymes are also expected to be a good tool for the estimation of the drug metabolism and the toxicity of drugs in humans. To show an example of the usefulness of such cell lines, we established new cell lines expressing CYP3A7, a form of cytochrome P450 specifically present in human fetal livers.