By the use of flow cytometric techniques, this prospective, randomized crossover study was designed to analyze intradialytic granulocyte reactive oxygen species (ROS) formation in whole blood with complement-activating and noncomplement-activating hollow fiber membranes. Dialysis with a complement-activating membrane resulted in a 6.5-fold increase in granulocyte hydrogen peroxide production 15 min after dialysis initiation and remained significantly elevated (P < 0.01) through the first 30 min with this membrane in comparison to both predialysis values and simultaneous values with a noncomplement-activating membrane. Further studies demonstrated that blood obtained at 15 min with a complement-activating membrane generated significantly less granulocyte ROS production in response to Staphylococcus aureus incubation than blood obtained either predialysis or at the same time in dialysis with a noncomplement-activating membrane. Both complement-activating and noncomplement-activating dialysis membranes caused slightly decreased granulocyte responsiveness to phorbol myristate acetate. It was concluded that hemodialysis with complement-activating membranes results in increased granulocyte ROS production and decreased responsiveness to S. aureus challenge during the dialysis procedure. These results document the potential role of ROS in hemodialysis-associated pathology and susceptibility to infection.