A great deal of effort has gone into research on the mechanisms of action of chemical carcinogens. The action of the host on the carcinogen represents one approach--to determine what metabolic products may be involved. Study of the action of the carcinogen on the host requires a thorough comparison of changes wrought in cellular constituents by the carcinogen. Meshing of these two approaches increases the potential to discover the detailed mechanisms involved. Despite the increase in knowledge, many crucial molecular events and their consequences in the process of carcinogenesis still remain a mystery.