In the field of inflammation research the inducible nitric oxide synthase (iNOS) became an important pharmacological target, since overproduction of nitric oxide (NO) after induction of this enzyme seems to be associated with numerous pathological conditions. NO released from cells can be detected and quantified photometrically as its stable product nitrite by a simple colorimetric reaction (Griess reaction). The aim of our study was to investigate whether this method might be suitable for the bio-guided fractionation of anti-inflammatory plant extracts. For this purpose we assayed extracts as well as fractions of the roots of Curcuma zanthorrhiza Roxb, which contain the known iNOS inhibitor curcumin, and compared the obtained activity with their curcumin content. Furthermore, leaf extracts of Betula pendula Roth, to which defined amounts of curcumin were added, were examined to clarify the question whether chlorophyll might interfere with the test system. The presented results suggest that the Griess assay is indeed suitable to guide fractionation of plant extracts in order to isolate highly active compounds. Factors, however, which might restrict the broad application of this assay are the limited selection of solvents which do not interfere with the system and high contents of chlorophyll in plant extracts.