Mouse, chimeric, humanized and human monoclonal antibodies (MABs) are all in use for treatment of human cancer. Unconjugated antibodies have a complex mechanism of action, dependent on the nature of the target structure. Antibodies can activate the immune system (antibody-dependent cellular cytotoxicity [ADCC], complement-dependent cytotoxicity [CDC], induction of tumor immunity [idiotype network]). ADCC appears to be one of the most important immune effector functions. Antibodies may also induce apoptosis, cell cycle arrest, inhibition of cell proliferation as well as angiogenesis and metastatic spread. For most antibodies there is no clear dose-response relationship in vivo. The effect of antibodies can be enhanced by combination with chemotherapy and/or by agents which activate the immune system. The best therapeutic effect may be obtained if MABs are used early in the course of the disease. Rituximab (anti-CD20) was the first registered MAB for the therapy of follicular lymphoma. Impressive results have been seen in combination with CHOP chemotherapy (cyclophosphamide, doxorubicin, vincristine and prednisone) in follicular and high-grade lymphomas. In other non-Hodgkin's lymphoma subtypes, promising results are also seen in combination with chemotherapy. Trastuzumab (anti-Her2) is a breakthrough in the treatment of breast cancer in combination with chemotherapeutic agents. This antibody is also in clinical testing for adjuvant treatment. Alemtuzumab (anti-CD52) has shown impressive results both in refractory chronic lymphocytic leukemia and as up-front therapy. There are many other antibodies in late stages of testing for registration. Interesting MABs include cetuximab (anti-epidermal growth factor receptor [EGFR]), especially in combination with radiotherapy in head and neck cancer; ABX-EGF (anti-EGFR) in renal carcinoma; bevacizumab (anti-vascular endothelial growth factor) in several solid tumors. Antiepithelial cell adhesion molecule antibodies show promise in combination with chemotherapy in the adjuvant setting of colorectal carcinoma. It is estimated that about 20 antibodies will be in clinical use by the year 2010.