Disseminated malignancies continue to pose vexing problems for both patients and health care providers. Although localized malignant lesions can often be extirpated with surgery or radiation therapy, most metastatic neoplasms are difficult to cure despite systemic chemotherapy. Furthermore, the alopecia, nausea, myelosuppression, neurotoxicity, and nephrotoxicity that often accompany conventional chemotherapy are potent deterrents for many patients. Consequently, investigators and patients with cancer have long sought alternative therapies that might be effective while avoiding the dreaded side effects of chemotherapy. Immunologic interventions have been particularly appealing because they use “natural” components of the immune system to selectively eradicate malignant cells while leaving normal cells unharmed. Immunotherapeutic attempts have included “active” approaches, such as development of tumor vaccines, and “passive” or “adoptive” immunotherapy, in which components of the immune system (antibodies, interferons, interleukins, activated T lymphocytes) are produced exogenously before they are administered to patients with cancer. Of these strategies, monoclonal antibodies have achieved the most recent success and publicity.