The effect of aging and interval between primary and secondary treatment in two-stage carcinogenesis on mouse skin.

Two-stage carcinogenesis experiments on mouse skin (female ICR/Ha Swiss mice) were done by initiating mice at three age levels (6, 44, and 56 weeks) and promoting after a 2-week interval. In another series, mice were initiated at age 6 weeks, and three time intervals (2, 36, and 56 weeks) were used between initiation and promotion. The initiating agent was 7, 12-dimethylbenz(a)anthracene and the promoting agent was phorbol myristate acetate in all experiments. The results showed a general decrease in tumor production with increasing age at the time of promotion. However, the initiating effect persisted even when the interval between initiation and promotion was 56 weeks.