Experiments using newborn and older animals of 17 strains of mice and 2 strains of rats, studied for their susceptibility to 12 carcinogenic chemicals, are summarized. In considering all types of tumors, four investigations claim that newborn animals are more susceptible to carcinogenic chemicals than are older animals. However, in studies in which only one or two tissue(s) were investigated, lymphoreticular tissue (6 cases), lung (5 cases), liver (5 cases), and skin (1 case) yielded more tumors when the chemicals were administered neonatally than at older ages. On the other hand, subcutaneous tissue (3 cases), skin (3 cases), breast (2 cases), kidney (1 case), and vascular tissues (1 case) provided more tumors when the chemicals were given at older ages than at birth. Furthermore, in one study lung, and in another, lymphoreticular tissue developed more tumors when the carcinogen was given to 2-week-old animals instead of to younger or older animals. Five publications gave no information on age effect, and in one, the conclusions were obscure. Investigations using only newborns treated with chemicals are also enumerated.

A general evaluation, and specific comments for each experiment, are presented. Certain factors—anatomic, viral, and metabolic—whose involvement in carcinogenesis was shown to be modulated by age, evolved gradually from these studies, although their exact influences are still unknown. It is concluded that the issue is more complex than was anticipated, and is still open to controversy.

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This review was written while the author was an Eleanor Roosevelt Fellow in the Department of Experimental Biology, Weizmann Institute of Science, Rehovoth, Israel.

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