KENNETH A. FOON and ALTON C. MORGAN, JR. Passive immunotherapy using heteroantisera for the treatment of cancer in animals and humans has been studied for over 50 years. Attempts have been made to treat animal tumors with sera from immunized syngeneic, allogeneic, or xenogeneic animals. A number of studies of passive immunotherapy using heterologous antisera in humans have also been performed. These studies have generally been attempted in patients with large tumor burdens, and as would be expected, results have been transient at best. A wide variety of solid tumors as well as leukemias and...