Current treatment of Entamoeba histolytica infection requires the use of several agents that are effective at different sites of the body. Commonly administered agents such as nitroimidazoles have a high rate of gastrointestinal side effects and their use is restricted during pregnancy. In order to offer new choices, four macrolide antibiotics (erythromycin, clarithromycin, azithromycin, josamycin) and metronidazole were tested for their in vitro activity against E. histolytica. Ten clinically isolated strains from an endemic area (Santo Domingo, Dominican Republic) were tested after polyxenical culture. Protozoan viability was significantly reduced by josamycin after 24 and 48 hours of incubation at a concentration of > or = 50 mg/l, which was slightly higher than that of metronidazole (25 mg/l). No resistance to metronidazole was found. The antiamebic activity of azithromycin, clarithromycin and erythromycin was significant at drug concentrations > or = 100 mg/l. High doses of josamycin, which is a very well tolerated drug, may serve as a useful therapeutic agent in the presence of E. histolytica infection.