Activity in an in vitro assay with Giardia lamblia provided a test of the validity of a quantitative methodology used in an ethnobotanical survey of the Luo people of the Lake Victoria basin of Kenya and Tanzania. Forty-five taxa of remedies for gastrointestinal problems were reported by four or more independent informants and a log-linear model was used to calculate a statistical measure of informant consensus. Methanolic extracts of 21 of 36 taxa assayed were lethal or inhibited growth of Giardia trophozoites at 1000 ppm; 7 species were lethal at 500 ppm. Non-cathartic species are more likely to be active than cathartics. Lethal species of non-cathartics are reported by informants more frequently than non-lethal species although the lack of statistical significance did not provide satisfactory support for the validity of the quantitative methodology as a predictor of efficacious remedies.