A rapid colorimetric assay with the tetrazolium salt MTT and phenazine methosulfate (PMS) for viability of Entamoeba histolytica. 
A rapid colorimetric assay for viability of E. histolytica trophozoites using the tetrazolium salt MTT and phenazine methosulfate (PMS) was developed.  The MTT is normally reduced by dehydrogenases of viable cells and transformed to formazan, a colored compound; the reaction is catalyzed by PMS.  Trophozoites of E. histolytica were incubated in the presence of MTT and PMS during 15, 30, 45, 60 and 75 min, at 37 degrees C. Trophozoites killed by heat or cold were used as controls.  The formazan produced into the cells was extracted with isopropanol-HCl and the optical density (OD) was measured at 570 nm.  There was a linear relation between the number of viable trophozoites and the OD570 (r2 = 0.98).  The trophozoites killed by cold or heat did not reduce MTT.  The optimal time of incubation with MTT/PMS was 45 min.  The MTT/PMS assay for determining viability of E. histolytica trophozoites is reproducible, rapid and easy to perform and it could be useful to study cytotoxicity and susceptibility to drugs.