Inflammatory cells such as macrophages and T lymphocytes play an important role in vascular calcification associated with atherosclerosis and cardiac valvular disease. In particular, macrophages activated with cytokines derived from T lymphocytes such as interferon-γ (IFN-γ) may contribute to the development of vascular calcification. Moreover, we have shown the stimulatory effect of 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) on in vitro calcification through increasing the expression of alkaline phosphatase (ALP), an ectoenzyme indispensable for bone mineralization, in vascular smooth muscle cells. Therefore, we hypothesized that macrophages may induce calcifying phenotype, especially the expression of ALP in human vascular smooth muscle cells (HVSMCs) in the presence of IFN-γ and 1,25(OH)₂D₃. To test this hypothesis, we used cocultures of HVSMCs with human monocytic cell line (THP-1) or peripheral blood monocytes (PBMCs) in the presence of IFN-γ and 1,25(OH)₂D₃. THP-1 cells or PBMCs induced ALP activity and its gene expression in HVSMCs and the cells with high expression of ALP calcified their extracellular matrix by the addition of β-glycerophosphate. Thermostability and immunoassay showed that ALP induced in HVSMCs was bone-specific enzyme. We further identified tumor necrosis factor-α (TNF-α) and oncostatin M (OSM) as major factors inducing ALP in HVSMCs in the culture supernatants of THP-1 cells. TNF-α and OSM, only when applied together, increased ALP activities and in vitro calcification in HVSMCs in the presence of IFN-γ and 1,25(OH)₂D₃. These results suggest that macrophages may contribute to the development of vascular calcification through producing various inflammatory mediators, especially TNF-α and OSM.