The relation between the expression and activity of MMP-9 in C-reactive protein (CRP)-induced human THP-1 mononuclear cells and the activation of nuclear factor kappa-B (NF-кB) was studied to investigate the possible role of CRP in plaque destabilization. Human THP-1 cells were incubated in the presence of CRP at 0 (control group), 25, 50 and 100 μg/mL (CRP groups) for 24 h. In PDTC (a specific NF-кB inhibitor) group, the cells were pre-treated with PDTC at 10 μmol/L and then with 100 μg/mL CRP. The conditioned media (CM) and human THP-1 cells in different groups were harvested. MMP-9 expression in CM and human THP-1 cells was measured by ELISA and Western blotting. MMP-9 activity was assessed by fluorogenic substrates. The expression of NF-кB inhibitor α (IкB-α) and NF-кB P65 was detected by Western blotting and ELISA respectively. The results showed that CRP increased the expression and activity of MMP-9 in a dose-dependent manner in the human THP-1 cells. Western blotting revealed that IкB-α expression was decreased in the cells with the concentrations of CRP and ELISA demonstrated that NF-кB P65 expression in the CRP-induced cells was increased. After pre-treatment of the cells with PDTC at 10 μmol/L, the decrease in IкB-α expression and the increase in NF-кB P65 expression in the CRP-induced cells were inhibited, and the expression and activity of MMP-9 were lowered too. It is concluded that increased expression and activity of MMP-9 in CRP-induced human THP-1 cells may be associated with acti- vation of NF-кB. Down-regulation of the expression and activity of MMP-9 may be a new treatment alternative for plaque stabilization by inhibiting the NF-кB activation. The relation between the expression and activity of MMP-9 in C-reactive protein (CRP) -induced human THP-1 mononuclear cells and the activation of nuclear factor kappa-B (NF-κB) was studied to investigate the possible role of CRP In plaque destabilization. Human THP-1 cells were incubated in the presence of CRP at 0 (control group), 25, 50 and 100 μg / mL for 24 h. In PDTC (a specific NF-кB inhibitor) group , the cells were pre-treated with PDTC at 10 μmol / L and then with 100 μg / mL CRP. The conditioned media (CM) and human THP-1 cells in different groups were harvested. MMP-9 expression in CM and human THP -1 cells was measured by ELISA and Western blotting. The results of NF-κB inhibitor α (IкB-α) and NF-κB P65 were detected by Western blotting and ELISA respectively. The results showed that CRP increased the expression and activity of MMP-9 in a dose-dependent manner in the human THP-1 cells. revealed that IκB-α expression was decreased in the cells with the concentrations of CRP and ELISA demonstrated that NF-κB P65 expression in the CRP-induced cells was increased. After pre-treatment of the cells with PDTC at 10 μmol / L, the decrease in IкB-α expression and the increase in NF-κB P65 expression in the CRP-induced cells were inhibited, and the expression and activity of MMP-9 were lowered too. It is concluded that increased expression and activity of MMP-9 in CRP-induced human THP-1 cells may be associated with acti- vation of NF-кB. Down-regulation of the expression and activity of MMP-9 may be a new treatment alternative for plaque stabilization by inhibiting the NF-кB activation.