Abstract. A rapid microplate assay for the detection of potential compounds active against Entamoeba histolytica is described. The assay is based on the metabolic reduction of a tetrazolium salt by E. histolytica trophozoites as an indicator of their viability which may be measured photometrically. The method was validated by determining the dose-response characteristics of standard amoebicides and correlating optical density and cell number; it provides a convenient means of selecting potentially novel anti-amoebic compounds for subsequent testing in vivo.