Using phytohemagglutinin (PHA) and/or Antilymphocyte serum (ATS) that are considered to act only on T-lymphocytes, a study was made of the effects of these substances on pleque forming cells (PFC) for sheep red cells in spleen of mice and the antitumor activity of cancerbearing mice as well as the blastformation rate of mouse lymphocytes against PHA in vitro, and the following results were obtained. 1) From the fact that PFC proliferate in the presence of PHA but they decrease in number in the presence of ATS, PFC seem to be under the control of T-cells. 2) As the antitumor effects of cancer-bearing mouse lymphocytes is enhanced by PHA but it is decreased by ATS, it may be said that T-cells play a leading role in the antitumor immunity as efector cells. 3) Since MIF activity of cancer-bearing mouse is decreased markedly by ATS, it seems that T-cells play the most important role in producing MIF. 4) Since the blastformation rate of mouse lymphocytes against PHA decreases in the presence of ATS, it may be assumed that the lymphocytes undergoing blastformation are T-cells. Various roles of T-cells are now being clacified, while the present experiment is a step toward the clacification of such roles.