Abstract: The effect of thymic peptide (TP) on tumor metastasis and Lymphocyte proliferation in C₅₇BL/6 mice treated with anti-theta serum was studied. The results showed the nunber of pulmonary metastatic colonies was increased and the respone of spleen lymphocytes tO Con A. measured by ~3H-thymidine incorporation was suppressed significantly as compared with Control. Treating the mice with TP could decrease the nunber of pulmonary metastatic colonies and enhance lymphocyte transformation as compared with those only treated with anti-theta serum.