In this experiment, the authors obserbed the effect of Pretiron (TSH) on the transplantation and growth of three kinds of trnsplantable tumors of animals. 1) Ehrlich Ascites Tumor Mice weighing approximately 20 g were used. All mice were transplanted with Ehrlich Ascites Tumor intraabdominally. Pretiron (TSH) was intramuscularly injected daily for the period of 23 days, beginning 3 days prior to the transplantation of tumor. In the Pretiron (TSH) treated group, the growth of free ascites tumor cells was markedly inhibited and the survival days of these mice were prolonged (20-50 days). In the ascites of this group of mice, the infiltration of numerous lymphocyte-like cells was recognized. On the other hand, the survival days of the control group of mice were less than 20 days and the marked growth of free ascites tumor cells was obserbed. 2) Fibrosarcoma 58. The growth of transplanted tumor in the Pretiron (TSH) treated group is moderately inhibited. In the tumor tissue of Pretiron treated group, the more maked infiltration of lymphocyte-like cells was seen than iu the control group. 3) Mammary Carcinoma in R Strin of Mice The growh of transplanted tumors of Pretiron (TSH) treated group was markedly inhibited and thelmajority of these tumors disappeared. Histologicaly, numerous lymphocyte-like cells was found to infiltrating in the stroma of the tumor tissues. On the contrary, in the control group, the transplanted tumors grew well in all mice. From these findings it is concluded that Pretiron (TSH) has an inhibitory action on these tumors. Although the mechanism of antitumor action of Pretiron (TSH) is unknown, it is interestinh that Pretiron (TSH) promotes the infiltration of lymphocyte-like cells in tumor tissues as obserbed in this experiment.