MANY tumours induced in rodents by chemical carcinogens1 and by viruses2 are antigenic in animals of the same highly inbred strain and even in the primary autochthonous host3. It has, therefore, been suggested that an inadequacy of the immune system may be a factor in allowing the growth of an induced tumour. Much evidence has been obtained in favour of such a hypothesis. Newborn animals, which have an immature immunological system, are much more susceptible to the oncogenic effect of viruses4 and of minute doses of chemical carcinogens5 than are adult animals. Chemical carcinogens have been shown to interfere with the normal immunological defences of the host6. Thymectomy of neonatal mice7, rats8 and hamsters9 leads to some impairment of immunological function, particularly homograft reactivity, and renders animals more susceptible to the oncogenic effects of polyoma virus10 and 3,4-benzopyrene11. The experiments reported here were performed to test the oncogenic activity of another chemical carcinogen, 20-methylcholanthrene, in thymectomized mice. Inbred C57BL mice from the colony maintained at the Chester Beatty Research Institute were thymectomized or sham-thymectomized on the third day of life. At approximately five weeks of age 200 µg 20-methylcholanthrene (MC) in solution in 0.2 ml. tricaprylin was injected intramuscularly into the right thigh. The animals were examined weekly and sarcomata at the injection site were measured and recorded. Mice were killed once the sarcoma reached a size of 18-20 mm diameter, examined post mortem, and material was taken for histological investigation. The results are summarized in Table 1. Thymectomized mice were examined macroscopically and microscopically for thymus remnants, but the four mice in which these were found are not included in the table. All animals, except one sham-thymectomized mouse, developed sarcomata at the injection site.