The effect of plant extracts on D-glucose uptake by Ehrlich ascites tumour cells was examined. Among the 23 exracts of medicinal plants, five samples inhibited, and six samples activated, the uptake significantly. From one of the active plants, Lagerstroemia speciosa, two triterpenoids, colosolic acid and maslinic acid were isolated. Colosolic acid was shown to be a glucose transport activator. Scince this compound was known to have hypoglycemic activity, our simple in vitro bioassay method can at least be used as a first screening for anti-diabetic activity.