Interleukin 13 (IL-13) is a recently described protein secreted by activated T cells which is a potent in vitro modulator of human monocyte and B-cell functions. The data, reviewed here by Gerard Zurawski and Jan de Vries, shows that IL-13 shares biological activities with IL-4, their genes are closely linked in both the human and mouse genomes, and there is sequence homology between IL-13 and IL-4 proteins. Although the cloned IL-4 receptor protein (IL-4R) does not bind IL-13, it appears that the functional IL-4R and IL-13R share a common subunit that is important for signal transduction.