Adhesiveness of in vivo aged human erythrocytes, fractionated by a density gradient centrifugation with Percoll, was studied. (i) The dissociation of the erythrocyte aggregates by shear force were more difficult in aged cells than in young cells. (ii) Aged erythrocytes were more adhesive on the artificial erythrocyte membrane reconstituted on alkylated glass plate than young erythrocytes. The increased adhesiveness of aged erythrocytes, in addition to the decreased deformability, might contribute to the removal of aged erythrocytes in the reticuloendotherial system.