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Macrophage is the immune phagocytic cell, playing variety of immunological and inflammatory reactions. The macrophage activation has been extensively studied in vitro using culture media like Dulbecco's modified Eagle’s medium (DMEM) and Ham's F-12 medium (F-12) in response to bacterial infection, tumor development, cytokines and so on. In the study of macrophage activation during co-culture with EL-4 tumor cells, we found different phenotypes of J774.1 macrophage-like cell line in F-12 and DMEM, when treated with lipopolysaccharide (LPS) and interferon-g (IFN- g). Among these phenotypes, nitric oxide (NO) production with corresponding inducible NO synthase (iNOS) expression was remarkable, showing higher in F-12 than in DMEM. Besides, O₂^-generating activity and production of interleukin-1b (IL-1b) were also higher in F-12 than DMEM, although production of tumor necrosis factor-a (TNF- a) was higher in DMEM than F-12. RT-PCR analysis revealed significantly higher expression of mRNA of iNOS, IL-1 b, IL-18, IkBa in F-12, but higher that of p65 and p105 in DMEM after treatment with LPS + IFN-g, suggesting these differences being induced at the transcriptional levels. Through investigation of critical factor(s) in these culture media that influence the activation phenotypes of the macrophages, we found that sodium bicarbonate (NaHCO₃) concentrations in these culture media, 14 mM in Ham's F-12 and 44 mM in DMEM, were the key. Culture medium-induced differences in macrophage activation were also observed in RAW264.7 macrophage-like cell line and in mouse peritoneal macrophages. The recent studies suggested involvement of carrier (SLC) transporter gene expression and subsequent elevation of JAK/STAT signaling cascades in these NaHCO₃ responses. Taken together, these results provide evidence for the importance of NaHCO₃ in the culture medium in the macrophage activation in vitro, implying important insights to the NaHCO₃ concentration in vivo in the body of patients suffering from inflammation, tumor development or immune disorders where macrophage activation is involved.