Hemolytic transfusion reactions (HTR) are characterized by fever, shock, organ system failure, intravascular coagulation, and possibly death. The same findings may be associated with sepsis. Neutrophils have been implicated in the pathogenesis of HTR, although a mechanism for neutrophil activation has not been shown. In addition, the possible role that cytokines may play in HTR has not been investigated. We show that interleukin-8 (IL-8), a cytokine with chemotactic and neutrophil-activation properties, is produced in whole blood following addition of ABO-incompatible red blood cells, in a dose- and time-dependent manner related to the degree of hemolysis, and is inhibited by inactivation of complement. IL-8 production is accompanied by increased gene expression in the buffy coat. This observation has implications for the understanding of the pathogenesis of and for the treatment of HTR.