The substance P (SP)-preferring receptor neurokinin-1 receptor (NK-1R) has two forms: a full-length receptor consisting of 407 aa and a truncated receptor consisting of 311 aa. These two receptors differ in the length of the C terminus of NK-1R. We studied the undifferentiated and phorbol myristate acetate (PMA)-differentiated human monocyte/macrophage cell line THP-1 to investigate the expression and function of NK-1R. The expression of full-length and truncated NK-1R in this cell line was determined by using real-time PCR and immunofluorescence staining. Undifferentiated THP-1 cells expressed only truncated NK-1R. The differentiation of THP-1 cells with PMA to a macrophage-like phenotype resulted in the expression of full-length NK-1R, which was functionally accompanied by an SP (10−6M)-induced Ca2+increase. In contrast, the addition of SP (10−6M) did not trigger Ca2+response in undifferentiated THP-1 cells; however, SP did enhance the CCR5-preferring ligand RANTES (CCL5)-mediated Ca2+increase. When a plasmid containing the full-length NK-1R was introduced into undifferentiated THP-1 cells, exposure to SP triggered Ca2+increase, demonstrating that the full-length NK-1R is required for SP-induced Ca2+increase. The NK-1R antagonist aprepitant (Emend, Merck) inhibited both the SP-induced Ca2+increase in PMA-differentiated THP-1 cells and the SP priming effect on the CCL5-mediated Ca2+increase, indicating that these effects are mediated through the full-length and truncated NK-1R, respectively. Taken together, these observations demonstrate that there are unique characteristics of NK-1R expression and NK-1R-mediated signaling between undifferentiated THP-1 cells and THP-1 cells differentiated to the macrophage phenotype.