With cancer-bearing mice as the subjects of our study, we studied allogeneic inhibitory activity of lymphocytes which constitutes the center of the immunological surveillance system believed to be protecting mammalians from cancer, and obtained the following results. At first, methylcholanthrene-induced sarcoma (MC-sarcoma) was isografted to C(3)H mice subcutaneously on the back between scapulae. Then the regional axillary lymph nodes were taken out and lymph node cells were isolated from these nodes. These lymph node cells were mixed with JTC-11 cells derived from Ehrlich cancer cells in the ratio of 40:1, and the mixed cells were cultured for 240 or 48 hours. During this culture the increase in the number proliferating JTC-11 cells, counted every week, was taken as the allogeneic inhibitory activity of the regional lymph node cells. As a result it has been found that in the early stage of one to two weeks after tumor transplantation there can be observed an allogeneic inhibitory activity as strong as that of normal mouse lymph node cells, but by the third or fourth week such an allogeneic inhibitory activity of the regional axillary lymph node cells has been completely lost. In other words, it seems that in the case of progres sive cancer the allogeneic inhibitory activity decreases along with time.