At the annual meeting of the American Association for Cancer Research in Washington, three years ago, Dr. C. C. Little Expressed the belief that all experimental cancer research on animals should be repeated with controlled strains of animals from genetically well established stock. Having seen the material of Cook, Hieger and Kennaway (1, 2) in 1932, and being particularly impressed with the fact that 1:2:5:6-dibenzanthracene offered a stable and consistent carcinogenic agent, we believed that it was an ideal compound with which to work out this idea. We decided, therefore, to attack the problem from the angle of the response to painting with debenzanthracene, of a strain of mice having a naturally high incidence of spontaneous tumors as compared with a strain having no spontaneous tumors. Painting was chosen primarily because of our limited facilities and because Burrows (3, 4), Peacock (5), Maisin (6), Lacassagne (7), and others had already well established the injection method. We also wished to ascertain whether or not the efficiency of this new carcinogenic agent might be in any way affected by reticulo-endothelial block.