Since 1915, when Yamagiwa and Ichikawa succeeded in producing tar cancer experimentally, considerable progress has been made towards the elucidation of the chemical nature of substances capable of inciting malignant changes in cells. Outstanding contributions since then have been those of Bloch and Dreifuss (1921) and of Kennaway and his colleagues. From the work of Bloch and Dreifuss it appeared that the carcinogenic material in tar was present in the neutral, nitrogen-free, high-boiling fraction. Kennaway (1924, 1925, 1930) produced carcinogenic substances from acetylene and from other compounds under conditions which indicated that the active substances were hydrocarbons. Following the application of the fluorescence spectrum to this problem by Mayneord in 1927, Hieger (1930) noted that many cancer-producing materials showed similar fluorescence spectra. Thereupon a number of compounds which were found by the English investigators to have spectra similar to those of the cancer-producing tars were tested on animals. This led to the observation that 1:2:5:6–dibenzanthracene was capable of producing cancer in experimental animals. A large number of compounds 1 of this type were then synthesized by Cook and his collaborators; animal assays showed that certain of these compounds were highly active. In the earlier experiments of this group of English workers carcinomas were produced by applying solutions of the compounds to the skin of the experimental animals. Burrows, Hieger and Kennaway (1932) then made the significant finding that 1:2:5:6–dibenzanthracene could also produce tumors of connective tissue. Thereupon, experimental work with dibenzanthracene was undertaken in this laboratory.