Thymic extract showed antitumor effect to sarcoma mice with higher dose$(200{mu}g/mouse/day$ i.p., 4weeks) but not with low dose$(5{mu}g/mouse/day$ i.p., 6 weeks). Direct cytotoxicities were exhibited against sarcoma 180, L1210 and MOLT-4 by MTT assay. The spleen weight of mice were increased but the number of circulating lymphocytes were not increased after long-term(2 weeks) administration of thymic extract. Evaluating the mitogenesis by MTT assay. $\%$ absorbance of human lymphocytes was not increased by thymic extract. Cell cycle statistics of S phase and $G_2/M$ phase was not increased in the presence of that by PI staining. The formation of rosette was induced, irrespectively of exposure time short-term(l hour) and long-term(2 weeks). The population of mouse blood T-cell to bind Lyt2-antimonoclonal antibody and to $L_2T_4$ were increased after administration of thymic extract$(2-200{mu}g/mouse/day)$. From the above results, it is suggested that thymic extract exerts antitumor activity by stimulating T cells to differeniate in vivo but not in vitro.