Guinea pigs were given a series of intramuscular injections of Entamoeba histolytica antigens mixed with Freund's complete adjuvant. The antigens were sonicated whole amebae and fractions obtained from amebae separated by column chromatography and concentrated by passage through ultrafiltration membranes. After three immunizing doses the animals were challenged with intracecal injection of amebae and were killed 6 days later. Gross and microscopic observations were made, the degree of protection was noted and the antibody response was studied. Fraction I had the largest molecular weight and complete protection was obtained. Partial protection was noted with the other antigens; 70% with whole amebae, 64% with fraction III and 43% with fraction II. Ninety-two percent of unimmunized animals were infected. Unusual lesions were observed in the liver of the immunized animals and their significance is discussed.