The effects of endothelin-1 (ET-1) on erythrocytes from sickle cell disease (SCD) patients have been described, but mechanisms of ET-1 regarding primary erythrocyte functions remain unknown. ET-1 is a vasoconstrictor peptide produced by endothelial cells, and the expression of ET-1 is increased in SCD. The present study used ex vivo experiments with sickle cell erythrocytes, ET-1 and bosentan, a dual antagonist of ETA and ETB receptors. We performed a HbS (hemoglobin S) polymerization assay with three concentrations of ET-1 (1, 20 and 50 pg/mL) and bosentan (100 nM). ET-1 increased HbS polymerization at all concentrations, and this effect was suppressed by bosentan. For the deformability assay, RBCs were incubated on a Sephacryl column with the same concentrations of ET-1 and bosentan. ET-1 decreased deformability, and this effect was reversed by bosentan. To observe erythrocyte adhesion, ET-1 and bosentan were incubated with RBCs in thrombospondin-coated 96-well plate, which demonstrated that ET-1 decreased adhesion but that bosentan enhanced adhesion. We also assessed erythrocyte apoptosis and observed decreased eryptosis induced by ET-1, and these effects were inhibited bosentan. Thus, these findings demonstrated that ET-1 modulates HbS polymerization, erythrocyte deformability, adhesion to thrombospondin and eryptosis, and these effects were suppressed or enhanced by bosentan.