Publisher Summary This chapter discusses a broad array of lymphokines and monokines that have marked biological effects on a variety of cell types, including B and T lymphocytes, macrophages, and other cells. The migration inhibitory factors, the chemotactic factors, the mitogenic factors, the helper and suppressor factors, the lymphotoxins, and growth-promoting factors are discussed. Evidence is presented for single factors with multiple activities, a topic that has plagued investigators in this field. Cellular immune reactions are mediated by T lymphocytes, and the expression of these phenomena includes cutaneous delayed-type hypersensitivity, contact allergy, resistance to infection by facultative intracellular microorganisms, graft rejection, and tumor surveillance. These reactions result from complex interactions between T cells and B cells, T cells and other T cells, T cells and macrophages. Lymphokines are classified functionally according to their effects: inhibitory, stimulatory, or inflammatory. The chapter describes various soluble factors that are prime candidates for mediating various immunologic reactions, particularly those relating to cellular immunity. These factors can be produced by a variety of nonlymphoid sources. This implies a more general biologic role for lymphokines and monokines in host defense and other homeostatic mechanisms. The ability of cell types other than lymphocytes to produce lymphokine- and monokine-like factors provides a safeguard for the organism.