We recently described a novel chemoattractant receptor, provisionally named CMKRL1, which has turned out to be the first cloned leukotriene (LT) receptor. Present binding assays using tritiated LTB4 and isolated membranes from COS-7 cells, transiently transfected with cDNA encoding this receptor, yielded a linear Scatchard plot showing expression of only a single, high-affinity receptor population with a mean Kd of 2.1 nM and Bmax of 17.0 pmoles/mg protein. Sham-transfected cells exhibited no specific binding. LTB4 elicited concentration-dependent increases in intracellular calcium measured with Fura-2 in individual CHO cells stably expressing CMKRL1. No response was seen with sham-transfected control cells, or in calcium-free medium which suggests that calcium mainly originates from extracellular sources. The LTB4-induced cellular calcium increment was blocked in the presence of a monoclonal antibody, raised against a synthetic peptide corresponding to the extracellular tail of CMKRL1 and capable of visualizing the receptor by fluorescence immunocytochemistry. Taken together the analyses show that LTB4 is the endogenous ligand for CMKRL1 which is, thus, identical to the LTB4 receptor, designated BLTR according to the NC-IUPHAR nomenclature.