During the last decade a considerable body of knowledge has come into existence concerning a class of carcinogenic molecules chiefly represented by 4-nitroquinoline i-oxide. Original papers on this subject are numerous and widely scattered over many branches of science; it was felt that these papers should be reviewed and the knowledge brought together in one volume before it became too unwieldy. This we have attempted to do in this monograph. Our aim has been to include all relevant papers published to date, so that it may serve as an epitome of the present status of knowledge on this important subject. We have been fortunate in securing the cooperation of several colleagues who have contributed chapters, each dealing with one aspect of the subject. We have been doubly fortunate in that these contributors, like ourselves, were at one time or another members of the group belonging to the scientific staff of Dr. WARO NAKA­ HARA, Director of the National Cancer Center Research Institute, Tokyo, Japan.