1,25-Dihydroxyvitamin D₃(1,25(OH)₂D₃) and interferon-gamma(IFN-γ) are capable of regulating cells in immune system and are representative differentiation inducer/activator of monocyte/macrophage lineage cells. They may affect the proinflammatory and immune effector functions of monocyte/macrophage activated with immunostimulants such as bacterial endotoxin. The expression of HLA-D, costimulatory adhesion molecules such as ICAM-1(CD54) and Mac-1(CD11b), and secretion of IL-1β were used to evaluate the influence of 1,25(OH)₂D₃ and IFN-γ on human promyelocytic cell line THP-1 cells. IFN-γ markedly induces the expression of ICAM-1 and HLA-D, and 1,25(OH)₂D₃ induces the expression of Mac-1 to a lesser extent. LPS alone markedly induces the expression of ICAM-1 molecule without any increase of HLA-D expression. LPS-induced ICAM-1 expression is synergistically increased by IFN-γ priming, but fail to increase Mac-1 and HLA-D expression. IFN-γ, 1,25(OH)₂D₃, and LPS separately do not induce the secretion of IL-1β by THP-1, however, IL-1β secretion is synergistically increased when THP-1 is exposed to the combination of IFN-γ and LPS. LPS-induced IL-1βproduction by IFN-γ-primed cell is much higher when THP-1 have been previously exposed to 1,25(OH)₂D₃. In conclusion, these results show that expression of MHC class II and costimulatory adhesion molecules, and production of IL-1β by cells of monocyte/macrophage lineage is highly regulated and monocyte /macrophage differentiation is required for optimal proinflammatory and immune reaction in response to exposure of bacterial endotoxin.