A controlled chemotherapy trial of trachoma was carried out in a Tunisian oasis where the disease is highly endemic and frequently leads to loss of vision. Children 6 to 9 years of age with clinically active trachoma were divided into three groups. One of the groups was treated with chlortetracycline ophthalmic ointment (48 cases), one with erythromycin ophthalmic ointment (52 cases), and one with boric acid ointment (46 cases). Medications were administered twice daily six days a week for ten consecutive weeks. Both antibiotics were considerably more effective than boric acid in suppressing clinical disease for four weeks and 17 weeks after treatment. Similarly, both antibiotics considerably reduced the prevalence of trachoma agent (in immunofluorescent and Giemsastained smears) at four weeks after treatment, but a noticeable decrease at 17 weeks was found only in the erythromycin group, in which there was much less trachoma agent before treatment than in the other two groups.