Eighteen patients undergoing total hip replacement (n = 13) or knee arthroplasty (n = 5) due to osteoarthritis or osteoarthrosis were prospectively studied in an investigation of complement activation and anaphylatoxin release in association with reinfusion of aspirated wound blood. Twelve of the patients needed blood transfusions and received an average of 390 ± 75 ml (±SD) of autologous blood within 45 min. Plasma complement components, anaphylatoxins, and inhibitors were studied 1 min before and 15 min after the start of and 15 min after the completion of autologous transfusion. Samples also were taken from the collected blood, before and after passing it through a microporous filter. Blood gases and systemic complement samples were drawn simultaneously. There were no significant changes in systemic complement variables before, during, or after transfusion of autologous blood. However, in the aspirated blood, increased concentration of anaphylatoxins (C3a and C5a) and terminal complement complexes (TCC) were present (P< 0.001). There were no differences observed between samples drawn before and after filtration of the blood. The concentration of C5 was less in the collected blood than in the systemic blood (P< 0.05). No changes in blood gases were observed. This study demonstrated that post-operatively salvaged whole blood underwent anaphylatoxin formation and complement activation. However, after reinfusion of this blood, neither systemic complement activation nor clinical complications were observed.