The deliberate and selective in vitro growth of normal T cells was first described by Morgan et al. in 1976 (1). These investigators described the development of a system in which the initiation and long-term culture of normal T cells was strictly dependent on the continuous presence of conditioned medium from phytohemagglutinin (PHA)-stimulated human blood lymphocytes. The growth-promoting activity of their conditioned medium was undoubtedly due to the presence of the T-cell product we now know as interleukin-2 (IL-2). The discovery of IL-2 activity and the subsequent characterization of the biochemical and molecular features of IL-2 have dramatically influenced the development of many aspects of immunological investigations. As a T-cell growth factor, IL-2 has permitted the generation and maintenance of long-term cultures of antigen-specific T-cell clones. This has led to significant advances in our understanding of complex processes such as the specificity of MHC-restricted responses, 90 identification of the T-cell receptor, elucidation of the molecular events involved in cell-mediated cytolysis, and exploration of the potential of IL-2 as an immunotherapeutic and immunomodulating agent (2). In this chapter, we discuss the available information regarding the structure, molecular biology, and biology of IL-2 and the IL-2-receptor. We will also discuss the various pharmacological and toxicological effects associated with the in vitro and in vivo use of IL-2. We will not, however, discuss aspects of IL-2 that relate to: 1) the role of IL-2 in pathogenesis of animal diseases, 2) the role of IL-2 as an immunotherapeutic agent, and 3) the use of IL-2 as an adjuvant. These topics will be covered in other chapters. Most of the available information regarding many of the topics covered in this chapter is derived from studies with human and murine IL-2. We will use this information as the foundation for the discussion of corresponding information regarding IL-2 from various animal species.