['To determine if the complement system, a potent mediator of inflammation, contributes to haemolysis during red blood cell (RBC) storage.', 'RBCs in storage undergo structural and biochemical changes that may result in adverse patient outcomes post-transfusion. Complement activation on leukodepletion and during storage may contribute to the RBC storage lesion.', 'We performed a cross-sectional analysis of aliquots of leukoreduced RBC units, stored for 1-6\u2009weeks, for the levels of C3a, C5a, Bb, iC3b, C4d and C5b-9 [membrane attack complex (MAC)] by enzyme-linked immunosorbent assay (ELISA).', 'We observed that only MAC levels significantly increased in RBC units as a function of storage time. We also observed that the level of C5b-9 bound to RBCs increased as a function of storage time.'] Does membrane attack complex generation increase as a function of time in stored blood?