Mice. Four-week-old inbred male and female NMRI mice

Source of Compounds. DMBA was supplied by EGA-Chemie, Steinheim, Germany, and l,2,3,4-dibenzanthracene (1,2,3,4DBA) by Nutritional Biochemicals Co., Cleveland, Ohio. The labeled compounds thymidine-3H and uridine-3H were purchased from the Radiochemical Centre, Amersham, En gland. The cocarcinogenic compound Al was prepared by Dr. H. Bresch, Heidelberg, Germany. Mice. Four-week-old inbred male and female NMRI mice were obtained from Dr. Spiegel, Institut fur Versuchstierzucht, Hannover, Germany. Induction of Tumors. In order to demonstrate the carci nogenic activities of the compounds used, a series of experi ments were performed as described in a recent communication (21). The hydrocarbons, dissolved in 0.3 ml polyethylene glycol 400:dimethylformamide (24: 1), were administerd in single nontumor-inducing doses to 5-week-old mice. The ace tone solution of cocarcinogen Al was applied to the shaved skin on the necks of the animals twice weekly. The first dose of the cocarcinogen Al was administered 1 week after the per