As a consequence of interactions with the dialysis membrane, neutrophils are activated and undergo degranulation during hemodialysis using all types of membranes. Previous studies have suggested that the release of neutrophil granular proteolytic enzymes into plasma contributes to the protein catabolic state in the dialysis patient. The mechanisms by which intradialytic degranulation occurs are, however, unknown. Since complement is also activated during hemodialysis, it has been postulated that the complement activation product, C5a, mediates most of the intradialytic neutrophil alterations, such as degranulation and oxygen radical production. Evidence supporting this hypothesis is, however, lacking and clinical studies have found a poor correlation between complement activation and plasma levels of neutrophil granular proteins during hemodialysis. In order to understand the mechanisms of dialysis induced-neutrophil degranulation, we have used an in vitro model in which isolated neutrophils are stimulated by cuprophan membranes and plasma. Our results have shown that cuprophan membrane-induced degranulation is dependent on plasma, and is mediated by both complement-dependent and complement-independent mechanisms. We also showed that C5a alone is insufficient to account for the complement portion of the degranulating activity. Further, the noncomplement factor does not appear to be a previously described substance that is known to induce neutrophil degranulation. In this application, we propose to: i) determine the interactions between proteins that are bound to cuprophan membrane during plasma exposure and proteins that are present in the fluid-phase which result in neutrophil degranulation; ii) identity the complement proteins and neutrophil surface complement receptors that mediate the degranulation; iii) identify the noncomplement plasma factor that mediates the degranulation; iv) study the capacity of various types of commonly used dialysis membranes to support the activation of the noncomplement plasma proteins that stimulate neutrophil degranulation, and to examine if this capacity correlates with the complement activating potential of the membranes. We anticipate that these studies will elucidate the mechanisms by which hemodialysis induces neutrophil activation and degranulation. In addition, they will define a novel complement-independent mechanism by which neutrophils can be stimulated to degranulate that may be applicable to other inflammatory states.