Albendazole, a benzimidazole carbamate commonly used for the treatment and control of intestinal helminthic infections, is also useful for the treatment of giardiasis. Therefore, it is of interest to determine whether the drug has activity against other intestinal protozoa, such as E. histolytica. The present results demonstrate that albendazole inhibits the growth of E. histolytica trophozoites in axenic cultures and induces fine structural changes such as polyribosome aggregation and loss of cytoplasmic vacuoles at concentrations up to 10 micrograms/ml. The viability of E. histolytica trophozoites was not affected by the drug. In contrast, lower concentrations of albendazole showed dramatic effects on G. lamblia trophozoites. These included loss of adhesiveness, striking modifications of the overall morphology of giardias, disassembly of the ventral disk, and loss of viability after prolonged treatment. The results provide further evidence on the potent antigiardial activity of albendazole and indicate that, at the concentrations used, the drug has no antiamebic activity.