Several mechanisms result in suppression of the immune response. For exam ple, either T cells (1-3), B cells (4), or macrophages (5-7) can mediate antigen nonspecific suppression. However, discussion of these phenomena is beyond the scope of the present review. In this chapter we consider only T-cell sup pressor systems specific for the antigen that stimulated their development. In addition to helper T lymphocytes, which permit development of immune responses to T-dependent antigens, there is a distinct subclass of T cells that suppress specific immune responses. The existence of antigen-specific sup pressor T cells following antigen administration was originally demonstrated by Gershon & Kondo (8) and has been studied subsequently in many labo ratories. Suppressor T cells are responsible for many phenomena associated with immunological tolerance. In addition, suppressor T cells are an essential component of a homeostatic mechanism that keeps the course and size of specific immune responses under control.