Despite testing since the mid-1900s, only in the past three years have some monoclonal antibodies provided sufficient efficacy and safety data to support regulatory approval as cancer therapy. Adjuvant-edrecolomab monoclonal antibody was approved in Germany after demonstration of a statistically significant 32% improvement over observation alone in the seven-year mortality rate for patients with colorectal cancer. Similarly, trastuzumab monoclonal antibody combined with chemotherapy prolonged the median time to the progression of breast cancer compared to chemotherapy alone. Unconjugated monoclonal antibodies investigated for the treatment of hematologic malignancies include anti-idiotype, CAMPATH-1, and rituximab. Rituximab was the first such therapy approved in the United States for relapsed or refractory low-grade or follicular B-cell non-Hodgkin's lymphoma after demonstration of an overall response rate of 48% and a duration of response of 11.7 months. The radioisotope-conjugated monoclonal antibodies tested as therapy include anti-B1, LYM-1, LL2, anti-CD33, and ibritumomab tiuxetan. Clearly, the full potential of immunotherapy still lies ahead.