Considerable progress has been made in the past year in clarifying the structure-function relationships that are central in controlling T lymphocyte proliferation. Once a cDNA probe for the IL-2 receptor is cloned, all the primary reagents needed for detailed study of the functional consequences of IL-2 binding will be available and one anticipates even more accelerated progress. Studies designed to explore the therapeutic potential of IL-2 are embyronic at present but there is sufficient reason to believe that such replacement therapy is now feasible and will be beneficial in some instances.