1. Swiss and C3H male mice received one, two, or three feedings by stomach tube of 9,10-dimethyl-1,2-benzanthracene, 3,4-benzpyrene, 20-methylcholanthrene, or 1,2,5,6-dibenzanthracene, and then weekly feedings of croton oil for 30 weeks. The tumor yield in the forestomach was consistently higher than when the carcinogen treatment was followed by 30 weekly feedings of the solvent alone.

  2. In the control series, in which croton oil was fed weekly for 30 weeks without previous carcinogenic hydrocarbon feeding, papillomas of the forestomach also developed in 14–16 per cent of the animals.

  3. The increased tumor yield in the series in which the carcinogenic hydrocarbon treatment was followed by croton oil could be attributed in most cases to a summation of carcinogenic action of the hydrocarbon and the croton oil, though, in one case, there was a suggestion of a true promoting action on the part of the croton oil over and above that attributable to their combined carcinogenic activities.

  4. The need for different promoting agents, in place of croton oil, and for other changes in technic for the study of the two-stage mechanism of carcinogenesis in the stomach is discussed.

  5. Polyethylene glycol-400 (a carbowax possessing both lipophilic and hydrophilic properties), which was used throughout as solvent for the various agents, was found to produce a decided anticarcinogenic effect at the promoting phase (i.e., when given repeatedly after cessation of the feeding of the carcinogen), though not when serving as solvent for the carcinogenic hydrocarbons (i.e., when acting concurrently with the carcinogen).

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This work was supported in part by grants from the Damon Runyon Fund (1951–53).

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