Factors that modify tissue responses to chemical carcinogens in different organ systems are studied to evaluate their contribution to changing susceptibilities to carcinogenesis in certain tissues during prenatal and post-natal development. Tissues and organ systems currently under study include the nervous system, kidneys, and liver. Modifying processes now being studied inlcude excision repair of DNA, the changing cellular susceptibility to carcinogens during different stages of the cell cycle, and the role of the enzyme gamma-glutamyltranspeptidase as a determinant of susceptibility to the carcinogen azaserine. Factors anticipated for future study include the induction and genetics of enzymes metabolizing carcinogens.