Introduction Some twenty years ago@ various procedures for the catalytic cracking and reforming of petroleum came into commercial use in certain large units of the American oil industry. Out of these processes come side streams and products of crude petroleum in which the reformed and original hydrocarbons are so distributed as to create mixtures with new physical and chemical characteristics. Certain of these@ particularly in the high boiling ranges@ proved capable of producing cancers of the skin of mice. Further@ these materials contain appreciable amounts of polycyclic hydrocarbons identical or similar in their chemical structure to those incriminated earlier as having a possible causal relationship to human skin cancers and generally accepted today as potential carcinogens. The recognition of these facts@ suggesting that the hazard of skin cancer in petroleum workers might assume significant proportions@ led the Medical Advisory Committee of the American Petroleum Institute too recommend a comprehensive study of this problem. After the necessary and appropriate negotiations@ the Kettering Laboratory was authorized to undertake this investigation. Chemical and biological testing of various fractions began in 1948.