In 1976, Morgan et al first reported the presence of a T-cell growth­ promoting activity in the supernatants of activated T-cell cultures; they denoted this T-cell growth factor (TCGF) (1). In subsequent years, TCGF was characterized as a distinct biochemical entity and found to be a 15,500dalton glycoprotein (2). TCGF was purified to homogeneity (3), molecu­ larly cloned (4), and cDNAs expressed in eukaryotic cells. The entire gene has now been sequenced (5, 6) and localized to human chromosome 4 (7). TCGF, or interleukin-2 (IL-2) as it is now denoted, has played a key role in the study ofT-cell biology. This lymphokine has permitted both the long­ term growth and the cloning of hurnan and murine T-cell lines displaying helper and cytotoxic T-cell function (8). Furthermore, possible therapeutic effects of IL-2 in various malignancies and immunodeficiency states are now under study. IL-2 is not produced by resting T cells; however, following activation with antigen or mitogen in the presence of the monokine interleukin-l, IL-2 is rapidly synthesized and secreted (Figure 1) (9). Following activation,