Biological response modifiers improve the body's ability to fight cancer by immunostimulation. Although a century has passed since the first attempt was made to stimulate the host immune system against cancer, only the past decade has witnessed the scientific use of biological response modifiers. Recent advances in tumour immunology have enabled the development of specific agents targeted against cancer cells. Various biological response modifiers include monoclonal antibodies, interferons, interleukins, tumour necrosis factor, colony stimulating factors and anticancer vaccines. Monoclonal antibodies directed against tumour-specific agents have been approved for the treatment of breast cancer (trastuzumab), non-Hodgkin's lymphoma (rituximab) and for the diagnosis of certain cancers (oncoscint). Interferons are indicated for the treatment of certain leukaemias and Kaposi's sarcoma to inhibit tumour proliferation and angiogenesis. Interleukin-2 is the most widely studied interleukin, and is used for immunostimulation in metastatic renal cell carcinoma and malignant melanoma. Haematopoletic growth factors are often combined with chemotherapy and radiotherapy to restore bone marrow function and treat complications such as infection and bleeding. Thalidomide, which suppresses tumour necrosis factor-alpha production and has antiangiogenic properties, is currently under evaluation in several cancers. Various anticancer vaccines are being developed using tumour cells, carbohydrates, peptides and heat-shock proteins as antigens. DNA-based vaccinations and the use of recombinant bacteria and viruses to deliver antigens or the DNA coding for them are also being investigated. However, the optimum choice of antigen, delivery vector and adjuvant, and administration regimen for some of these biological response modifiers are still being investigated.