The influence of small amounts of croton oil applied to the skin of mice for periods up to 83 weeks was tested. After a latent period of 8–10 weeks, papillomas appeared in the more susceptible strains, and usually by 20–24 weeks at least 50 per cent of the mice bore one or more tumors. Carcinomas appeared between 28 and 36 weeks, and the incidence was as high as 25 per cent of the effectual total4 by 1 year. Other strains of mice were tested and found to be less susceptible. The response to croton oil was much greater, and the latent period was shorter, when the mice had been previously treated with a subcarcinogenic amount of 9,10-dimethyl-1,2-benzanthracene.

*

This work was supported in part by grants from the Alexander and Margaret Stewart Trust Fund and from the American Cancer Society on recommendation of the Committee on Growth of the National Research Council.

We are indebted to S. B. Penick and Co. for generous gifts of croton oil.

A preliminary report of a part of this work was presented at the Sixth International Cancer Congress, São Paulo, Brazil, July 23–29, 1954 (12).

4

Effectual total is defined as the number of mice alive at the time the first malignant tumor was detected.

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