Growth of syngeneic tumours in unimmunized newborn and adult hosts.

The post-natal development of "natural" resistance of Balb/c mice to the challenge of syngeneic tumours was studied using injections of various doses of live neoplastic cells into untreated animals of increasing age, from neonatal to 12 weeks.The minimum quantity of neoplastic cells capable of inducing tumours increased in parallel with the age of the animals. Immunodepression with whole body irradiation with X-rays reduced the resistance offered by adult mice to tumour challenge to that of the neonate. The relationship of the increase in resistance to tumour challenge with the development of the animals' own immune response during the course of post-natal growth is discussed.