Anti-HER2/neu therapy of human HER2/neu expressing malignancies such as breast cancer has shown only partial success in clinical trials. To expand the clinical potential of this approach, we have genetically engineered an anti-HER2/neu human IgG3 fusion protein containing interleukin-2 (IL-2) fused at its carboxyl terminus. Anti-Her2/neu IgG3-(IL-2) retained antibody and cytokine related activity. Treatment of immunocompentent mice with this antibody fusion protein resulted in significant retardation in the subcutaneous (s.c.) growth of CT26-HER2/neu tumors suggesting that anti-HER2/neu IgG3-(IL-2) fusion protein will be useful in the treatment of HER2/neu expressing tumors. We also found that fusing IL-2 to human IgG3 results in a significant enhancement of the murine anti-human antibody (MAHA) response.