In several of our previous publications we have referred to experiments with chrysene which indicated that this substance had a low degree of carcinogenicity for mice, and that in combination with oleic acid, itself apparently capable only of inducing benign tumour formation, we had a mixture which had a definitely greater activity than either of its constituents applied separately. Prior to our experiments Kennaway (1) had tested chrysene on mice, and came to the conclusion that it was inactive. Quite recently Barry and Cook (2) report experiments which lead them to doubt whether our results were not really due to impurities in our chrysene, and to question whether this particular hydrocarbon is per se carcinogenic. Subsequent to our investigations on chrysene and oleic acid, Kennaway and his collaborators (3, 4, 5, 6), in a brilliant series of experiments, reported the isolation and synthesis of several pure hydrocarbons which have a carcinogenic activity for mice immeasurably greater than that of chrysene. The importance of these observations was obvious to all workers on cancer research, and they were of particular interest to us as they supported our experiments with chrysene, and our ideas with reference to the possible significance of the phenanthrene nucleus in the induction of cancer (7). The earliest experiments on chrysene in these laboratories were carried out with a commercial sample which was definitely yellow (8, 9). The main impurity in this case would probably be naphthacene (2, 3 benzanthracene), which, it is now thought, is the “chrysogene” of Fritzsche (10), the contaminating substance giving the yellow colour to coal-tar chrysene. Naphthacene appears to be non-carcinogenic (9).