The future role of the immunomodulators in medical practice is yet to be defined. The key question is whether these new substances will bring remarkable progress in transplantation or in the treatment of such conditions as cancer, AIDS, and autoimmune diseases, or whether they will be of only minor adjunctive importance. As background to the discussion of immunomodulating agents, the immune system is explained, with emphasis on the roles of T and B lymphocytes, macrophages, phagocytes, human leukocyte antigen and the complement system. Special attention is given to the cytokines, particularly the lymphokines. The immunomodulators can be divided into three main groups: immunosuppressive agents, such as FK 506 and rapamycin; immunostimulating agents, of which BCG vaccine is most important; and the remaining immunomodulators, which include the biological response modifiers. The last group, which encompasses the colony-stimulating factors (GM-CSF, G-CSF, and M-CSF), the interleukins, the interferons, and the tumour necrosis factors, is described in detail. Innovative research and medical applications of these cytokines, including indications, contraindications, and adverse reactions, are discussed. The role of monoclonal antibodies against endotoxins is also described.