...many allergic patients exhibit high levels of circulating antigen-specific IgE but never have anaphylaxis, whereas other subjects with low concentrations of specific IgE in the blood can experience anaphylactic shock.1 Endothelin-1 (ET-1), a 21-amino-acid naturally occurring peptide with potent vasoconstrictor activity, has been reported to exacerbate certain allergic reactions in rodents. MCs are thought to represent a critical effector cell in IgE-dependent anaphylaxis.1 Prior functional assays and expression analyses of various MC populations demonstrated that ETA can be expressed by connective tissue-type MCs, including peritoneal MCs.5,6,9 However, immature bone marrow-derived cultured mouse mast cells (BMCMCs), which have some similarities to mucosal-type MCs, are largely unresponsive to ET-1 and have very low levels of receptor expression.6 We confirmed, using flow cytometric analysis, the substantial expression of ETA and weak expression of ETB on C57BL/6 mouse peritoneal MCs (Fig 1, A), as well as the absence of detectable surface ETB and very weak expression of ETA on C57BL/6 mouse BMCMCs (Fig 1, B).