Interleukin-13 (IL-13) is a recently discovered immunoregulatory cytokine. The cellular sources of IL-13 and the regulation of its expression are largely unknown. Here we show that human basophils produce IL-13 in response to IgE-receptor (IgER) crosslinking, IL-3, IL-3 plus C5a, but not C5a alone. Human basophils express IL-13 in a restricted manner since, apart from IL-4, no other cytokines encoded on the cytokine gene cluster (IL-3, IL-5, and granulocyte macrophage-colony-stimulating factor [GM-CSF]), are induced. Highest levels of IL-13 are formed after IgE-independent activation leading to a prolonged secretion of IL-13. The response to IgER-cross-linking is more transient preferentially inducing IL-4, IL-3 is a unique cytokine regulating IL-13 production by human basophils: Among a large number of cytokines tested, only IL-3 is capable of directly inducing IL-13 expression. Furthermore, although some IL-13 is produced in response to C5a in the presence of IL-5, GM-CSF, IGF-1 or IL-1 beta, IL-3 is by far the most effective. IL-13 production was blocked by actinomycin D and cycloheximide and conditions leading to IL-13 release also lead to the induction of IL-13 mRNA. This study supports an important immunoregulatory role of human blood basophils, owing to their capacity to simultaneously express IL-13 and IL-4 in a restricted manner.