Neutrophil function in patients with acute renal failure (ARF) and serum inhibitors of neutrophil chemotaxis associated with ARF were studied. Neutrophil function was examined by measurement of chemotaxis, phagocytosis and superoxide anion generation by neutrophils. Chemotaxis, phagocytosis and superoxide anion generation by neutrophils in ARF patients showed significantly (p<0.01) reduced quantities (84.4±34 counts/field, 320±195 counts/100cells and 0.78±0.21 nmol/min/10⁵cells, respectively) in comparison with normal healthy subjects (117±26.6, 582±132, 1.41±0.25, respectively). Sera from ARF patients were studied before and after hemodialysis. Hemodialysis reduced the inhibition of chemotaxis and phagocytosis of normal neutrophil by sera from ARF patients. When sera of these ARF patients were fractionated by Sephacryl S-300 column chromatography, the specific fractions responsible for reducing chemotaxis were found in the molecular weight range of 2001500 daltons, which were not found in normal healthy subjects. The rate of inhibition of chemotaxis by fractionated inhibitors was reduced approximately 30% by hemodialysis. These findings suggested that hemodialysis for ARF patients contributes to the preservation of neutrophil function.