Antigens in extracts of two strains of Entamoeba histolytica (HT-31 and HK-9) were studied by two-dimensional immunoelectrophoresis (2D-IEP) and 32 and 20 different antigenic components were detected, respectively. Cross-reactions between heterologous systems showed a large number of shared antigens between the two strains. Extracts were fractionated by Sephadex G-200 chromatography. Of the fractions obtained, fraction II of both strains was found to have the most reactive antigenic components. Precipitin patterns of fraction II from HT-31 and HK-9 with sera from patients showed similar 2D-IEP profiles and suggested that in this fraction antigens common to both strains exist which elicit antibody in individuals with E. histolytica infections.