Crude extract preparation The Croton spp. root was identified and provided by Assoc. Prof. Chavi Yenjai, Faculty of Sciences, Khon Kaen University, while the other plants were collected from local market in Khon Kaen in 2007. The selected plants were collected, washed, and prepared into small pieces. The dried plants were macerated with extracting solvent such as methanol, ethanol, 50% ethanol-water or water. The organic extracts were filtered and evaporated under vacuo. The aqueous extract was freeze dried. The obtained crude extracts were freshly prepared in methanol as a stock solution and used for the study.