In his introduction to the Ciba Foundation Symposium on "Immunopotentia tion," Sir Peter Medawar stated that, "for the past twenty years the control of the immune response has been virtually equated to immunosuppression because the great goal of applied immunology has been the transplantation of tissues," and that, "with the discovery of tumor immunity-the focal point of immuno logical control has changed from immunosuppression to immunpotentiation and correspondingly the great prize of applied immunology has become the preven tion and control of malignant growth." Corynebacterium parvum is now univer sally recognized as being the most potent presently known immunopotentiator. C parvum is at the center of almost all papers presented at this conference and it has been analyzed from various viewpoints: taxonomy, pharmacology, immunopotentiation, stimulation of host resistance to tumor invasion in experi mental models and in human therapeutics. Almost all data reported in this volume are original and recently obtained. The discussions were spontaneous, stimulating, and highly enriching, and the results already established and re ported here are in many respects unique. Discussions at the conference revealed many other aspects which require further exploration. Nonspecific immunopotentiation has strongly penetrated into immunology, immunopathology, and, recently, oncology. Undoubtedly, this area of research will witness important developments in the near future. This book offers the first synthesis of presently available knowledge and emphasizes future prospects.