Observations on Chemically Induced Regress- ing Tumors of Mouse Epidermis 1,2

SUMMARY-Tumors were induced in the skin of Swiss mice by repeated applications of 7,12-dimethylbenz[alanthracene (DMBA) (one-stage ex-periments) or by a single application of DMBA Followed by repeated applications of enhancing agents (two-stage experiments). The rate of tumor regression, Followed accurately, was lower in the one-stage than in the two-stage experiments. In the two-stage experiments, the rate of regression was proportional to the tumor yield and to the duration of the experiments. In these experiments, a shortened period of enhancement led to a marked increase in the rate of regression; the rate was also affected by the site of treatment but not by the enhancing agent. It was concluded that regressions were not due to the repeated applications of DMBA, nor to the repeated applications of enhancing agents. Since inability of the individual tumors to sustain continued growth under different experimental conditions was vari-able, a likely mechanism of regression of this type of tumor was the initial potential tumor Focus induced by the carcinogen. Such Foci had a variabl