The HER2/neu receptor is a protooncogene that is overexpressed in approximately 30% of human breast cancers. Therapy with a recombinant humanized monoclonal antibody to HER2/neu (Herceptin) has shown efficacy in large clinical trials, both as a single agent and also when used in combination with chemotherapeutic agents/drugs. However, only 15-20% of patients with HER2-overexpressing tumors will respond to this treatment, and responses appear to be limited to those tumors with the highest expression of HER2. Attempts to enhance the actions of Herceptin have been hampered by our limited knowledge of its mechanism of action. We have recently demonstrated that co-stimulation of natural killer (NK) cells with Herceptin-coated breast cancer cells and interleukin-12 results in the production of large quantities of interferon-gamma (IFN-g) and other immunomodulatory cytokines with anti-tumor actions. Based upon these findings, we have recently initiated an NCI-sponsored Phase I trial in which Herceptin is administered in combination with interleukin-12 to patients with HER2-overexpressing malignancies. This trial is based upon the theory that the host immune response to biologically relevant monoclonal antibodies may be significantly enhanced in the presence of interleukin-12.