We measured membrane fluidity of polymorphonuclear leukocytes (PMN) from 14 uremic patients on hemodialysis by the excimer-forming lipid technique with pyrenedecanoic acid using flow cytometry. Membrane fluidity of PMN was significantly lower in the uremic patients during the predialysis period. During hemodialysis, progressive normalization in membrane fluidity was observed. Cross-incubation studies indicated that this observation is ascribed to factors in the patients' serum and that they can be removed by hemodialysis. When sera of these patients were fractionated by Sephadex G-25 column chromatography, the specific fraction responsible for a decreased fluidity was found in the low-molecular-weight fraction. Uremic patients have an increased risk of infection, which may be partly due to altered membrane fluidity of their PMN.