The binding to normal and sialidase-treated human erythrocytes and lymphocytes of four 125I-labeled lectins [Maackia amurensis hemagglutinins (MAM and MAH), Ricinus communis hemagglutinin (RCH), and Bauhinia purpurea hemagglutinin (BPH)] was studied in detail. The quantitative inhibition assays against the lectin binding to the cells were also performed with various glyco-proteins and glycopeptides as inhibitors. The comparison of the inhibition constants of the inhibitors thus obtained with the association constants of the lectins to the cells permitted estimation of the relative receptor activities of cell surface glyco-proteins toward the lectins.