By subcutaneous inoculation of N, N'-dimethylnitrosourea to adult male C3Hf/Bi mice once a week for 10 consecutive weeks the authors studied the correlation between immunological functions and histological changes in lymphatic tissues at the latent period of thymic lymphoma whose development is known to occur in 100 per cent. As a result, it was found that PFC of the spleen to sheep erythrocytes decreased to about one third the normal level by two weeks, and to one
tenth by 8 weeks after initial inoculation of this compound. Hemolysin and hemagglutinin titers of the serum became less than 1 : 2 after 6 weeks and later. As for histological changes in the thymus, disappearance of
lymphocytes became marked by 2 weeks, and there appeared tumor cells by 8 weeks. Also the peripheral lymphocytes as well as the total spleen cells decreased in number along with increase of the frequency of inoculation of N,N'-dimethylnitrosourea.
These results seem to suggest that the immunosuppressive effect of carcinogen facilitates the development and proliferation of tumor cells possessing tumor specific antigenicity in the course of N, N'-dimethylnitrosourea-
carcinogenesis.</p
