Amoeba proteus is proposed as a low-cost multi-purpose biochemical tool for screening and standardizing cytotoxic plant extracts with possible application in the laboratories of developing countries. Advantages and limitations of this test are examined and different mathematical treatments (probit analysis versus curve fitting to Von Bertalanffy and Hill functions) are investigated. Known anti-cancer (doxorubicin, daunorubicin, dacarbazine, 5-fluorouracil) and antiparasitic (emetine, dehydroemetine, metronidazole, cucurbitine, chloroquine) drugs were tested using this method and only metronidazole appeared inactive. Application of this model to Euphorbia hirta established that a 100 degrees C aqueous extraction of fresh aerial parts allows efficient extraction of active constituents and that drying the plant material before extraction considerably reduces activity.