Entamoeba histolytica antibody levels before and after treatment were determined by the indirect hemagglutination test (IHA) on 274 persons in Cali, Colombia. The tests gave positive results for 9% of 75 asymptomatic persons who passed cysts, 7% of 118 uninfected controls, 81% of 168 persons with amebic colitis, and 87% of 31 with extraintestinal amebiasis. In the latter groups antibodies persisted for at least 6 months after treatment. The antibodies measured in the test appeared not to be protective as reinfection was frequent among persons with relatively high titers. No relation was evident between antibody response and concurrent parasitic or bacterial infection, anemia, age or sex of the host, or previous amebic infection.