1) T cell population and blastogenic activity against both phytohemagglutinin (PHA) and pokeweek mitogen (PWM) were assayed in twenty patients, whose malignant neoplasma had been eradicated by surgery before more than six months, when the first clinical evidence of the recurrence was observed during the period of postoperative control. A deterioration of the blastogenic activity of the lymphocyte to PHA was highly significant, compaired with that in healthy subjects and patients who have had radical surgery for their neoplasm but showed no evidence of the recurrence. It was noted that this deterioration occurred regardlessly of the interval between the surgery and the recurrence and also of the mode of recurrent lesion. No remarkable differences were observed in the T cell population and the blastogenic activity to PWM among these three groups. 2) Swiss mice were undergone subcutaneous autotransplantation of isolated tumor cells from their own methylcholanthrene-induced leg sarcoma after surgical removal of the tumor. The establishment of the autotransplantation and the growth of the tumor was daily observed and the blastogenic activity of the splenic lymphocyte to PHA was assayed 14 days after the tumor implantation. Neither a period before the tumor take nor cumulative incidence of the take were correlated with the blastogenic activity of the splenic cell. However, the individual size of the tumors 14 days after the transplantation was markedly smaller in mice having the blastogenic activity higher than the mean value in the group than in those having the activity lower than the mean. Thus the reverse correlation between the tumor size and the blastogenic activity ratio were statistically significant (p<0.01). An additional skin homograft performed simultaneously with the tumor inoculation resulted in the apparent increase in the number of the mice showing the blastogenic ratio higher than the mean in the non-grafted control, and this caused further inhibition of the tumor growth, showing also the significant reverse correlatio between the tumor size and the blastogenic ratio (p<0.01). B. C. G. inoculation, before, at and after the tumor transplantation, caused marked decrease in the mitogenic activity of the splenic cell, but the tumor growth was significantly detarded and the reverse correlation between the tumor size and the blastogenic ratio among the individual mice was preserved as in the above two experiments. In mice administered with Cephranthin, daily for 14 successive days starting with the tumor transplantation, the tumor growth inhibition of slight grade was observed as late as 14 days following the tumor inoculation and no significant effect on the blastogenic activity of the splenic cell was observed, and though the reverse correlation above was affirmed it was not highly significant (p<0.05). 3) From the results mentioned above, the author intended to conclude that the reverse correlation between tumor growth and blastogenic activity of the lymphoid cell does exist and even the nonspecific immunopotentiation contribute to bestow the ability for the lymphoid cell to suppress the tumor growth by potentiation of the blastogenic activity, even in mechanism of establishment of autotransplantation of malignant tumor in the subject whose malignant neoplasma was eradicated, as in cancer bearing hosts. Additionally, the possibility that deterioration of the blastogenic activity of the lymphoid cell may play a role of triggering effect for dormant cancer cell to obtain a growing ability was presumed and a possibility of significant contribution to this mechanism of cell- mediated cancer immunity other than that can be assessed by mitogenic activity of the lymphoid cell was also discussed.