Sequestration of 111In-labeled neutrophils (polymorphonuclear leukocytes--PMNs) in the lungs during sham dialysis was studied dynamically using a gamma camera. Five animals were pretreated with cobra venom factor, naja naja (CVF), to achieve depletion of the complement system, mainly C5a. Seven animals were studied without pretreatment with CVF. Central hemodynamics and gas exchange were studied as well as neutrophil function using luminescence and aggregation. In the control group there was a significant sequestration of neutrophils in the lungs, which reached a maximum between 15 and 17 min after the start of hemodialysis. The peripheral neutrophil count decreased concomitantly. These changes were virtually absent in the CVF-treated group. PMN aggregation and luminescence were abolished in CVF-treated animals. Both pulmonary artery pressure and pulmonary vascular resistance increased significantly in the control group, whereas in the CVF group these parameters remained unchanged. There were no differences in blood gases, platelet count, or hematocrit between the groups. The results clearly indicate that the activation of neutrophils within minutes after the start of hemodialysis is greatly dependent on C5a activation because it can be abolished by C5a depletion. This activation is accompanied by changes in the pulmonary circulation with increases in pressure and resistance.