Summary. Complement activation and generation of pro-inflammatory cytokines occur during storage of blood components. Prestorage leucocyte filtration of platelet concentrates and red cells diminishes the accumulation of leucocyte-derived cytokines during storage, however, transfusion reactions are not eliminated. We investigated inflammatory mediator release during storage of plasma and whole blood and the effect of prestorage leucocyte filtration of plasma. Twenty-four blood units were collected from healthy blood donors and stored for 35 days. Eight units were stored as whole blood, eight units as plasma and eight units as prestorage filtered plasma. Samples were collected weekly for analyses of potassium, leucocytes, free plasma haemoglobin, complement activation (C3a and SC5b-9) and pro-inflammatory cytokines [interleukin (IL)-6, IL-8 and tumor necrosis factor (TNF)-α]. Elevated levels of C3a and SC5b-9 were registered in filtered plasma, from the beginning of storage. C3a levels increased during storage. There was a higher rate of change during storage in C3a (P < 0·01) and SC5b-9 (P < 0·05) in plasma compared with filtered plasma. Interleukin (IL)-8 is released in whole blood. The cytokine levels generated in plasma and filtered plasma were low. Complement activation is present in whole blood, plasma and filtered plasma during storage. Prestorage filtration of plasma activates the complement cascade but does not influence cytokine generation.