Tumor-associated macrophages (TAMs) have the potential to induce both immune activation and immune tolerance. Recent studies have indicated that in breast cancers the pro-tumor role of TAMs is dominant. We induced rat peritoneal macrophages with rat breast cancer cell-conditioned medium and analyzed signal transducer and activators of transcription 3 (Stat3) activities of the cells. Then these cells were transfected with Stat3 decoy oligonucleotides (ODNs) and were stimulated by lipopolysaccharide (LPS). The results demonstrate that induced macrophages displayed a reduction of cytotoxicity and antigen-presenting function in comparison with control but transfection with Stat3 decoy ODNs enhanced cytotoxicity and antigen-presenting function of the macrophages. Furthermore, injection of induced macrophages promoted tumor growth accompanied by immunosuppression in the rat tumor models, but injection of induced macrophages transfected with Stat3 decoy ODNs led to retarded tumor growth accompanied by immune activation. The data suggest that immunosuppressive activities of TAMs correlate with over-activated Stat3 signaling of the cells and disruption of Stat3 activity of TAMs can enhance rat immune response to breast cancer.