Biotherapy has emerged as the fourth modality for the treatment of cancer. Biologic response modifiers (BRMs) alter the host-tumor relationship through immunomodulation to affect a biologic response. Currently, two BRMs, alpha-interferon and interleukin-2, have had clinical success in treating cancer and have gained approval by the U.S. Food and Drug Administration. Toxicities associated with BRMs are complex and often involve multiple systems. Understanding these toxicities and their corresponding pathophysiologic manifestalions aides nurses in their assessment and management of patients receiving BRMs.