Abstract Biologic therapy of cancer is defined as treatment that produces antitumor effects through the action of natural host immune mechanisms or the administration of natural animal substances. Biologic therapy is considered to be a potentially important fourth modality for the treatment of cancer alongside surgery, radiation therapy, and chemotherapy. Increased application of this form of cancer treatment will result from a better understanding of the basic aspects of host defense mechanisms against cancer and significant technologic developments that have made molecules available in quantities large enough for use in human patients. Although the field of biologic therapy remains relatively early in its development, there are many in vitro and in vivo examples of the successful application of biologic therapy to human cancer. Moreover, several recombinant biologic therapeutic agents in current use are considered standard therapy for selected malignant diseases. Future research in this field will focus on improved modulation of the human immune system and is anticipated to improve the current therapeutic armamentarium used against cancer.