In order to clarify the precise cellular mechanism of hemodialysis neutropenia, we examined the changes in the viscoelasticity of peripheral blood neutrophils using both the micropore and the microchannel filtration methods, and the changes in the neutrophil surface expression of Mac-1, L-selectin and sialyl Lewis X and the cytoplasmic expression of the actin filaments using a flow cytometric analysis during a dialysis session. Five patients with chronic renal failure were selected who showed a nadir leukocyte count in peripheral blood at 30 min after the initiation of the dialysis session. The neutrophil count also reached a nadir at 30 min and thereafter returned to almost the predialysis level by 180 min. Both the micropore filtration time and the microchannel passage time, which reflect the viscoelasticity of the peripheral blood neutrophils, correlated inversely with the neutrophil count. At the nadir of neutropenia, the neutrophils were observed to have become both adhesive and viscoelastic. The actin filaments in the neutrophil cytoplasm gradually increased in number from the start of dialysis, reaching a peak level at 30 min, and thereafter decreasing to predialysis levels. The Mac-1 expression continuously increased up from 30 min until the end of dialysis. The L-selectin expression first decreased at 15 min, but thereafter returned to predialysis levels within 60 min. The SLe<sup>x</sup> expression did not change throughout the course of the session. These results thus indicated the neutrophil counts during a dialysis session to inversely correlate with the viscoelasticity of the neutrophils expressed by the micropore filtration time or microchannel passage time, which possibly depends on the contents of cytoplasmic actin filaments. In addition, the shedding of L-selectin from neutrophil surface may also be involved in the first step of hemodialysis neutropenia.