Biological response modifiers (BRMs) are agents or approaches which can modify the biological response of the host to tumors and thereby are expected to augment the resistance to development or progression of cancer. Recent advances in the technology of genetic engineering and monoclonal antibodies have led to rapid progress in this field. There is an increasing number of genetically engineered cytokines, which appear promising for cancer treatment and are becoming available for clinical trials. These include the interferons, leukin-2, tumor necrosis factor, and colony-stimulating factors. For development of optimal therapeutic protocols with these and a variety of immunomodulatory agents, it appears necessary to develop a detailed understanding of the possible mechanisms of their antitumor effects, and to determine the optimal dose and schedule for altering the antitumor effector mechanisms. BRMs would also be expected to be quite useful in the prevention of cancer. Indeed, in a variety of animal tumor model systems, potent protective effects can be demonstrated. However, the strategies for clinical application of such information have yet to be adequately worked out. One more immediate application of this approach is for the prevention of metastatic spread of tumor cells. BRMs, which stimulate natural killer cell activity, have been shown to strongly protect against dissemination of tumors, and clinical strategies for this important aspect of cancer treatment are being developed.