On the administration of methylcholanthrene (MCA) to mice there can be observed a decrease in the plaque forming cells (PFC) from early stage, and in the post-administration week 5, at the time considered to be of precancerous stage the decrease in PFC number is most marked, being at the minimal level of 55.6% as compared with that of the control (untreated mice). In other words, the administration of MCA reduces the PFC number of spleen cells in mice and it markedly suppresses the immunological activity of normal mice. This indicates that MCA is appreciably involved in the immunity to carcinogenesis. On the other hand, the activity of allogeneic inhibition in the precancerous stage is maintained during the period between MCA administration and the time of cancer development. However, the allogeneic inhibition activity decreases by 8 weeks after MCA administration, the time when the tumor has grown to the size macroscopically visible. This finding suggests that the activity of the allogeneic inhibition is associated more with the proliferation rather than with oncogenesis.