Complement activation occurs during hemodialysis, and its intensity depends on the type of dialyzer and whether it is new or reused. Neutrophil degranulation also occurs during hemodialysis with release of lactoferrin, myeloperoxidase and elastase. However, it is unclear whether this event is induced by complement activation and whether it is attenuated by reuse. We examined complement activation and neutrophil degranulation during 10 consecutive hemodialyses using the same cuprophane dialyzer. Also the effect of rinsing the latter with 25% human albumin was studied. The rise in plasma C3a and C5a was markedly higher (p less than 0.01) during the first than the second use. Plasma levels of lactoferrin and myeloperoxidase increased significantly (p less than 0.01) during the first use, and levels were not affected by reuse. In contrast, plasma elastase increased with the first use and decreased with each subsequent use. Treatment of the dialyzer with albumin did not affect the magnitude of rise in plasma levels of C3a or lactoferrin but was associated with a significant reduction in plasma elastase. The data show that neutrophil degranulation is not dependent on complement activation and that the two processes could be dissociated.