Previously, we designed and synthesized a new NF-kappaB inhibitor, dehydroxymethylepoxyquinomicin (DHMEQ). In the present research we looked into the effect of DHMEQ on the activation of macrophages, especially on the phagocytotic activity of cells of the mouse macrophage-like cell line RAW264.7. DHMEQ inhibited lipopolysaccharide (LPS)-induced NF-kappaB activation by inhibiting its nuclear translocation from the cytoplasm. It also inhibited the expression of inducible NO synthase (iNOS) and nitric oxide (NO) production induced by LPS and interferon-gamma. Using enzyme-linked immunosorbent assays (ELISAs) we showed DHMEQ to inhibit LPS-induced secretion of IL-6, IL-12, interleukin-1beta (IL-1beta), and TNF-alpha. Furthermore, DHMEQ also inhibited the phagocytosis of fluorescently labeled Escherichia coli by RAW264.7 cells treated with LPS or IL-1beta, thus being the first evidence for the involvement of NF-kappaB in the regulation of phagocytosis by use of this inhibitor. Deletion of p65 by siRNA also inhibited the phagocytosis. DHMEQ inhibited the LPS-induced but not IL-1beta-induced phagocytosis of glass beads, indicating that activation of not only NF-kappaB but also Toll-like receptor 4 (TLR-4) is essential for the phagocytosis of E. coli. Previously we found that DHMEQ inhibited type 2 collagen-induced rheumatoid arthritis and the growth of various human carcinomas in mice. It is thus likely that inhibition of macrophage activation is involved in the mechanism of these anti-inflammatory and antitumor activities of DHMEQ in mice.