Initiation and promotion at different ages and doses in 2200 mice. II. Decrease in promotion by TPA with ageing

Using the data described in Paper I, we compare the effects of the same treatment timings and doses given at different ages. Initiation with DMBA at 68 weeks of age, followed 3 weeks later by TPA, has a significantly (P less than 0.0001) less rapid effect on subsequent tumour incidence than does initiation at 8 or at 48 weeks of age, followed 3 weeks later by TPA. We suggested that this is chiefly due not to changes in the numbers of cells initiated by DMBA, but rather to a decrease in the promotional efficacy of TPA in ageing mice.