TUMOUR necrosis factor-α (TNF-α)/cachectin is a multifunctional cytokine that has effects in inflammation, sepsis, lipid and protein metabolism, haematopoiesis, angiogenesis and host resistance to parasites and malignancy^1–3. TNF-α was first described in activated macrophages^1–3, but certain mouse or rat mast cell populations (reviewed in refs 4,5) and some in vitro-derived human cells with cytochemical features of mast cells-basophils⁶ may also contain products similar to TNF-α. Here we present evidence that resident mouse peritoneal mast cells constitutively contain large amounts of TNF-α bioactivity, whereas cultured, immature mast cells vary in their TNF-α content. IgE-dependent activation of cultured or peritoneal mast cells⁷ induces extracellular release of TNF-α and augments levels of TNF-α messenger RNA and bioactivity. These findings identify mouse mast cells as an important source of both preformed and immunologically inducible TNF-α, and suggest that release of TNF-α by mast cells may contribute to host defence, the pathophysiology of allergic diseases and other processes dependent on TNF-α.