The molecular characterization of cytokine receptors has progressed rapidly over the past 5 years as a result of availability of radiolabeled cytokines, as well as the identification or creation of cell lines that express significant numbers of receptors at the cell surface. This explosion in research effort has led to establishment of multiple cytokine-receptor gene families and the realization that inhibition of cytokine function at the level of ligand-receptor interaction may be an important area for therapeutic drug development.