Chinese medicinal plants were examined regarding inhibition of NF-B1 and COX-2 gene expression. 68 plants were screened in an in-vitro assay with LPS-stimulated THP-1 cells. A reproducible decrease of NF-B1 mRNA level was observed for Aristolochia debilis, Asarum species, Cassia species, Chrysanthemum indicum, Rehmannia glutinosa and Ziziphus spinosa extracts. Cell viability assay (XTT) revealed cytotoxic effects of A. debilis and A. species extracts. Ziziphus spinosa seed extract showed the highest anti-inflammatory potential and was subjected to bioactivity-guided fractionation. Two pure compounds were isolated: the quaternary aporphine alkaloid magnoflorine and a second compound, of which the structure could not be completely elucidated. Magnoflorine, however, showed no activity in gene expression assays. Several fatty acids as well as betulinic acid described from Z. spinosa seeds were also not active. So, the compounds responsible for the anti-inflammatory activity still need to be identified. Within the study, NF-B1 and COX-2 assays were revised in order to obtain higher reproducibility of the results. Method development included differentiation of THP-1 to macrophage-like cells with PMA, LPS-stimulation of gene expression, determination of positive controls quercetin (IC50=25.13M) and dexamethasone (IC50=3.30nM), and the selection of a reference gene (GAPDH) for real-time PCR. Investigations of flavonoids in NF-B1 assay showed some structure-activity relationships. Within the tested compounds, flavones showed the highest inhibition, followed by flavonols, flavanones and flavanonols, whereas the tested flavanols and flavonol glycosides were inactive and isoflavones rather increased the mRNA level. Apigenin, chrysin, chrysoeriol, luteolin, kaempferol and rhamnetin showed a concentration dependent effect on NF-B1 gene expression. XTT assay indicated that activity of some flavones may in part be due to cytotoxic effects on THP-1 cells.