Myelopeptides are a new class of endogenous bioregulatory peptides. Each has its own definite immunoregulatory activity and differs in their final effects and mechanisms of action. Myelopeptides act as immunocorrectors and thus prevent the development of immunodeficiency states. They have an antitumor effect which contributes to the retention of active immune defense in the tumor-bearing body. The ability of myelopeptides to induce terminal cell differentiation in hemopoietic disorders (in particular, in leukemia) is demonstrated. The protective effect of a single myelopeptide in bacterial contagion of animals, which is due to the stimulation of macrophage phagocytosis, is established. In this book, the concept of the general biological value of cytokines, including peptide endogenous regulators, is discussed. Myelopeptides can be considered as possible mediators of inter-system links, in particular, mediators of neuro-immune interaction.This book is intended for those interested in medical and biological problems: lecturers and students in medical and biological universities, scientists and researchers from medical and biological laboratories, and other professionals involved in medical and biological research.