Abstract Cysteinyl leukotrienes (CysLT) are potent inflammatory lipid mediators which are derived from 5-lipoxygenase activity. CysLTs mediate some of the pathophysiological responses associated with asthma such as bronchoconstriction, vasodilation and increased microvascular permeability, increased mucus secretion, decreased mucociliary clearance, eosinophil migration, and increased eosinophil survival. We now report that peripheral blood CD34+ progenitor-derived human mast cells and the human mast cell line (LAD2) express both CysLT receptors (CysLT1R and CysLT2R), the G protein-coupled receptors that bind CysLTs. LTC4, LTD4 and LTE4 activated LAD2 to produce MIP-1β and MCP-1 and production of these chemokines was inhibited by the CysLT1R inhibitor, monelukast. LAD2 activated by IgE/anti-IgE produced CysLT (980 + 55 pg/million cells) but LAD2 activated by substance P, a neuropeptide that also binds a G protein-coupled receptor, did not produce CysLT. Montelukast had no effect on SP-mediated LAD2 degranulation. CysLT did not induce human mast cell degranulation but montelukast inhibited FcεRI-mediated human mast cell degranulation by approximately 20% suggesting that autocrine production of CysLT potentiated FcεRI-dependent degranulation. IL-4 (10 ng/mL) in the presence of stem cell factor (SCF; 100 ng/mL) significantly upregulated expression of both CysLT1R and CysLT2R (16% and 32% respectively) and potentiated mast cell responses to LTE4 by up to 25%. Overall, these results show that human mast cell activation and degranulation is subject to autocrine CysLT-induced signaling which can be blocked by CysLT1R inhibitors such as montelukast. Funded by the Investigator Initiated Studies Program (IISP) from Merck and Company, Inc.