Mast cells (MC) play a central role in extrinsic allergic reactions such as asthma and may participate in other inflammatory and fibrotic processes. However, with the exception of immunoglobulin E (IgE) receptor-dependent stimulation, no secretagogues of human lung MC have yet been described. It is also unclear whether mediator release can be regulated by certain cytokines as demonstrated previously in basophils and other human inflammatory effector cells. Here, we show that the c- kit ligand (KL), a recently identified stem cell growth factor, at concentrations 10-100 times lower than that required to promote cell proliferation, enhances the release of histamine and leukotriene C4 in response to IgE receptor crosslinking of human lung MC. KL does not induce mediator release per se, but increases the sensitivity of MC to anti-IgE receptor stimulation and also enhances mediator release to maximally effective concentrations of anti-IgE receptor antibody. By contrast, a large number of cytokines examined, including the mast cell growth factors/agonists in rodents, interleukin 3 (IL-3), IL-4, IL-9, and nerve growth factor, were ineffective in this respect. These findings suggest a unique role of KL in regulating effector functions of human mucosal MC.