Blood monocytes and lymphocytes, a readily available source of human tissue, are used to assess in vitro an individual's ability to metabolize benzo(a)pyrene, a model compound of the class of polycyclic aromatic hydrocarbon carcinogens. A mammalian mutagenesis system using Chinese hamster V-79 cells will be used to assess the ability of human blood monocytes and lymphocytes and rat liver and mouse cells to activate polycyclic hydrocarbons to active mutagens.