cDNA cloning has revealed the presence of at least three distinct human receptors for macrophage inflammatory protein-1α (MIP-1α) and RANTES: C-C chemokine receptor (CCR) 1, 4, and 5. To clarify the physiological role of CCR1, we prepared specific antibodies to CCR1 by immunizing rabbits with recombinant glutathione-S-transferase (GST) fused with its NH₂-terminal portion. The resultant antibodies stained positively 293 cells transfected with CCR1 cDNA but neither parental cells nor cells transfected with CXCR1 [interleukin-8 (IL-8) receptor type A] cDNA, confirming its specificity. Immunofluorescence analysis revealed that peripheral blood lymphocytes and monocytes but not neutrophils express CCR1. Positive staining of transfectants, monocytes, and lymphocytes was inhibited by the GST protein fused with the NH₂-terminal portion of CCR1, further indicating that this antibody recognized the NH₂-terminal portion of CC CKR1. A majority of CD3⁺, CD4⁺, CD8⁺, or CD16⁺ peripheral blood lymphocytes but not CD19⁺ lymphocytes expressed CCR1. Among CD4⁺ peripheral blood lymphocytes, CD45RO⁺ cells expressed a larger number of CCR1 compared with CD45RO^-. Moreover, CD34⁺ cells in human bone marrow as well as cord blood were uniformly stained with this antibody. Furthermore, the antibody inhibited calcium mobilization in CCR1 transfectants stimulated with human rMIP-1α, suggesting that its NH₂-terminal portion is critically involved in ligand binding or signaling. Finally, the antibody partially inhibited monocyte chemotactic activities of human rMIP-1α, suggesting that CCR1 is a functional receptor for MIP-1α on human peripheral blood monocytes.