The cysteinyl leukotrienes (CysLTs) are important mediators of human asthma. Pharmacologic and clinical studies show that the CysLTs exert most of their bronchoconstrictive and proinflammatory effects through activation of a putative, 7-transmembrane domain, G-protein–coupled receptor, the CysLT₁ receptor. The initial molecular characterization of the CysLT₁ receptor showed by in situ hybridization, the presence of CysLT₁ receptor messenger RNA (mRNA) in human lung smooth-muscle cells and lung macrophages. We confirmed the results of these in situ hybridization analyses for the CysLT₁ receptor, and produced the first immunohistochemical characterization of the CysLT₁ receptor protein in human lung. The identification of the CysLT₁ receptor in the lung is consistent with the antibronchoconstrictive and antiinflammatory actions of CysLT₁ receptor antagonists. We also report the expression of CysLT₁ receptor mRNA and protein in most peripheral blood eosinophils and pregranulocytic CD34⁺ cells, and in subsets of monocytes and B lymphocytes.