The sera of four patients with amoebic liver abscess and two patients with diarrhea caused by Entamoeba histolytica were tested on "Western blots" prepared from E. histolytica strains A3, SFL-3 and HK-9 separated by SDS-PAGE. IgG antibodies in the sera from patients with liver abscess were found to react with several "major" (i.e. strongly staining) band of approximate molecular weights 150-250 kD, 100 kD, 80 kD, 60 kD, 35 kD, 30 kD, 20 kD, and 5-15 kD and the 150-250 kD group of bands. A comparison of the patterns of reactivity obtained with these sera and strains A3, SFL-3 and HK-9 would indicate that some of these bands, especially the 5-15 kD groups of bands might carry antigenic determinants specific for individual strains of amoebae. This 5-15 kD group of bands is completely pelleted at 30.000 g and could therefore be associated with the cytoplasmic membrane. The 150-250 kD group of bands consists of smaller subunits of 40-65 kD molecular weight linked by disulfide bridges; the respective proteins are sensitive to digestion with elastase but not with trypsin, and seem to carry antigenic determinants containing sugar residues as evidenced by their sensitivity to treatment with NaJO4.