The cysteinyl leukotrienes (cys-LTs) LTC(4), LTD(4), and LTE(4) are a class of peptide-conjugated lipids formed from arachidonic acid and released during activation of mast cells (MCs). We now report that human cord-blood-derived MCs (hMCs) express the CysLT1 receptor, which responds not only to inflammation-derived cys-LTs, but also to a pyrimidinergic ligand, UDP. hMCs express both CysLT1 protein and transcript, and respond to LTC(4), LTD(4), and UDP with concentration-dependent calcium fluxes, each of which is blocked by a competitive CysLT1 receptor antagonist, MK571. Stably transfected Chinese hamster ovary cells expressing the CysLT1 receptor also exhibit MK571-sensitive calcium flux to all three agonists. Both hMCs and CysLT1 transfectants stimulated with UDP are desensitized to LTC(4), but only partially to LTD(4). Priming of hMCs with IL-4 for 5 days enhances their sensitivity to each agonist, but preferentially lowers their threshold for activation by LTC(4) and UDP (approximately 3 log(10)-fold shifts in dose-response for each agonist) over LTD(4) (1.3 log(10)-fold shift), without altering CysLT1 receptor mRNA or surface protein expression, implying the likely induction of a second receptor with CysLT1-like dual ligand specificity. hMCs thus express the CysLT1 receptor, and possibly a closely related IL-4-inducible receptor, which mediate dual activation responses to cys-LTs and UDP, providing an apparent intersection linking the inflammatory and neurogenic elements of bronchial asthma.