In this study, in vitro inhibitory effects of 33 ethanol extracts obtained from 24 plant species (representing 11 different families) on cyclooxygenase-1 (COX-1) were evaluated. The plant materials selected for this study have been used in aboriginal medicine in Australia and traditional medicine in China for the treatment of various diseases that are considered as inflammation in nature, e.g. asthma, arthritis, rheumatism, fever, edema, infections, snakebite and related inflammatory diseases. All of the selected plants, with one exception, showed inhibitory activity against COX-1, which supports their traditional uses. The most potent COX-1 inhibition were observed from the extracts of Acacia ancistrocarpa leaves (IC(50)=23 microg/ml). Ficus racemosa bark, Clematis pickeringii stem, Acacia adsurgens leaves, Tinospora smilacina stem and Morinda citrifolia fruit powder exhibited inhibition of COX-1 with the IC(50) of 100, 141, 144, 158 and 163 microg/ml, respectively. Aspirin and indomethacin used as the reference COX-1 inhibitors in this study inhibited COX-1 with IC(50) of 241 and 1.2 microg/ml, respectively. The findings of this study may explain at least in part why these plants have been traditionally used for the treatment of inflammatory conditions in Australian aboriginal medicine and traditional Chinese medicine.