This study evaluated the anti-giardial activity of chloroform, methanol and water extracts of 12 medicinal plants (39 extracts), commonly used as self medication by AIDS patients in southern Thailand. The plant extracts and a standard drug, metronidazole, were incubated with 2x10(5) trophozoites of Giardia intestinalis per millilitre of growth medium in 96-well tissue culture plates under anaerobic conditions for 24 h. The cultures were examined with an inverted microscope and the minimum inhibitory concentration and the IC50 value for each extract was determined. The chloroform extracts from Alpinia galanga, Boesenbergia pandurata, Eclipta prostrata, Piper betle, Piper chaba, Zingiber zerumbet, and the methanol extracts from B. pandurata and E. prostrata were classified as "active", i.e. with an IC50 of <100 microg/ml, whereas the chloroform extract from Murraya paniculata was classified as being "moderately active". This study shows that extracts from some medicinal plants have potential for use as therapeutic agents against G. intestinalis infections.