By injecting 1 mg of methylcholanthrene (MCA) subcutaneously on the back of C3H and Zb mice the immunological state in the host was studied in the course of tumor growth thus induced by MCA, from the following two aspects. One point the plaque forming cell number (PFC number) of the spleen against sheep blood red cells (SBRC), and the other is the allogeneic inhibtory activity of regional axillary lymph node cells in mixed culture with JTC-11 cells (a strain derived from Ehrlich) or HeLa cells as target cells. Our observations have revealed that PFC number of spleen cells of the cancer-bearing mice decreases continuously from the time immediately after MCA injection up to the time of tumor death. The allogeneic inhibitory activity of regional lymph node, in the case with JTC-11 cells as target cell, is marked just before the tumor onset and shortly after the tumor onset, but such an activity is lost with time lapse of over 10 weeks when tumor grows bigger. However, when HeLa cell strain (derived from human cancer of uterus) with a great histoincompatibility is used as target cells, the allogeneic inhibitory activity is maintained persistenly up to the terminal stage of cancer. In other words, it can safely be said that there occurs continuous decrease in the PFC, number during the entire course up to tumor death, and the allogeneic inhibitory activity, which is considered to be one of the immunological surveillance system, also becreases in a progressive cancer irrespective of carcinogenic processes.