To understand the mechanism of Entamoeba histolytica adhesion, we characterized the binding of trophozoites to human erythrocytes (RBC) in suspension by measuring the kinetics of amoeba-RBC complex formation. Adhesion was very efficient, since most of the amoebae were complexed with RBC after only 5 min at 37 degrees C in mixtures containing 10(4) amoebae and 10(6) RBC per ml; the adhesion rate depended on amoeba and RBC concentrations, but not on the A, B, and O human blood groups, and was maximal at 37 degrees C and pH 7.3 in the presence of 4 mM Ca2+ and 1 mM Mg2+. Adhesion was prevented if trophozoites were fixed with glutaraldehyde, but only decreased slightly if RBC were previously fixed; it decreased in the absence of glucose and was inhibited as a function of the concentration of cytochalasin B and of the metabolic inhibitors bathophenanthroline and 8-hydroxyquinoline. From these results we conclude that E. histolytica adhesion is an active process that depends on the amoebal cytoskeleton and metabolic energy and on the mobility of both amoebal and RBC surface ligands.