Background and Objectives: Retransfusion of shed blood collected after operation has become popular, but recent reports of side effects led to a search for possible causes. Materials and Methods: In a randomized study of 28 patients undergoing total hip arthroplasty, shed blood was collected in Solcotrans, Orth-Evac, and ordinary Redon drainage. Osmotic fragility was measured and electron-microscopic pictures of erythrocytes from selective samples were taken. Serotonin, prostaglandin E2 (PGE2), and histamine were measured with enzyme-linked immunosorbent assays. Results: Higher osmotic fragility of erythrocytes collected with Solcotrans appeared to be due to ACD which was used only with that system. Serotonin concentrations did not differ significantly. However, there was a great increase in histamine (Solcotrans 477.7, Orth-Evac 344.0, Redon drainage 453.1 nmol/ml) and PGE2 (Solcotrans 1908.3, Orth-Evac 1225.0, Redon drainage 2666.7 μg/ml) in shed blood compared with venous blood (histamine 9.5 nmol/l, PGE2 4.2 μg/ml). Conclusion: Unwashed wound drainage blood collected after operation contains levels of proinflammatory mediators that can account for the reported side effects.