The current study determines the possible antitumor and immunomodulatory effects of thymosin against the in vivo and in vitro growth of tumor-derived cell line in mice. Peritoneal phagocytes count, Ehrlich ascites tumor (EAT) cells, T- lymphocytes, and B- lymphocytes activities were determined. In addition, serum level of interleukin 2 (IL-2) and liver functions were measured. In animal testing, thymosin at doses of 0.50 and 1mg activated the phagocytic function of macrophages, as well as T- and B- cell function. Thymosin caused a marked shortage in the proliferation of EAT cells in the peritoneal fluid with dose 0.50g as compared with that of the corresponding control group. Furthermore, treatment with thymosin caused effectively elevate in serum level of IL-2, on the contrary reduce in serum levels of ALT, AST and total proteins. The size of solid Ehrlich tumor was significantly decreased, as measured morphologically with the doses 0.50 and 1 mg (P<0.01). These results confirmed that many biological activities attributed to thymosin and is as an adjuvant for immune enhancement.