genic Hydrocarbons : 1 : 2 - Benzanthracene , Chrysene , 1 : 2 : 5 : 6 †” Dibenzanthracene , and 20 - Methyicholanthrene

INTRODUCTION The summation or additive effects of carcino genie agents have received much attention in ef forts made to understand the action of carcinogens and the events that occur in carcinogenesis (5, 21). Chemical carcinogens, tumor-producing viruses, radiant energy, and trauma have been tried in various combinations (46) . Some agents showed summation, others did not, and a third group showed inhibition. The present report is concerned with combinations of chemical agents only. The earliest work with summation of chemical carcinogens was done with tars (5) or other mix tures. After the carcinogenic hydrocarbons were discovered in 1931, the first summation experi ment with these compounds was made by Hieger (24). He found that one strong carcinogen (1 :2: 5 :6-dibenzanthracene) could complete the car cinogenic action on skin begun by another (8:4benzpyrene). Lavik et al. reported that methyl cholanthrene, benzpyrene, and dibenzanthracene administered in subcarcinogenic doses could elicit tumors following methyicholanthrene preparation (85). Rusch et al. observed that combinations of chemicals were additive in carcinogenesis on mouse skin but that chemicals and ultraviolet light, Shope virus and ultraviolet light, and Shope virus and x-ray irradiation did not summate (46). No addition was observed by Jafféwhen two car cinogens of a different chemical nature were used (27, 28). Thus, neither urethan nor azo dyes were additive to methyicholanthrene. Stasney et al. ob served augmentation of liver tumors in rats fed 2-acetaminofluorene if estradiol or pregnant mare serum gonadotrophin were also administered (49). It is assumed that in those experiments where summation did not occur its absence was due to