Human IL-4 is a mature glycoprotein of 129 amino acids, mostly secreted by activated T cells. It is a pleiotropic cytokine which acts on T and B lymphocytes, monocytes, polymorphonuclears, fibroblasts and endothelial cells. In addition, it acts at various stages of cell differentiation, and its effects are also dependent on the cytokine environment. In particular, IL-4 blocks some of the effects of IL-2 whereas interferon-gamma blocks some of the effects of IL-4. In vitro and in vivo experiments in mouse and in vitro experiments in man have shown that IL-4 plays a crucial role in the induction of IgE production whereas interferons counteract this effect. Human IL-4 binds to a high-affinity receptor which is composed at least of one 130 kDa glycoprotein of 800 amino acids which is a member of the newly described hematopoietin receptor superfamily. IL-4 may prove useful as an anti-tumoral and anti-inflammatory agent.