Bioreactive substances including cytokines and lipids accumulate during storage of red blood cells (RBCs) but their clinical importance is uncertain. The goal of this study was to evaluate the effect of stored RBC supernatant on neutrophil activity in vitro. Packed RBCs (PRBCs) were collected and divided into two aliquots, one leukodepleted and the other nonleukodepleted. Plasma supernatant from PRBCs were collected on days 1, 8, 15, 29 and 35 and its effect on neutrophil expression of CD11b, CD16 and oxidative burst was measured by flow cytometry. Levels of tumour necrosis factor alpha (TNFα) and interleukin-8 (IL8) were also measured. The supernatant from PRBC units stored for greater than 15 days activated and primed neutrophils as evidenced by an increase CD11b and CD16 expression and oxidative burst. The greatest effect was seen in the oldest concentrates (35-day-old) (P < 0.008). Leukodepletion abrogated the effects of stored supernatant on CD11b and CD16 expression (P < 0.02) but did not reduce priming of the neutrophil oxidative burst (P > 0.1). Very low levels of IL8 and TNFα were detected in stored supernatants. Stored PRBC supernatant contains substances which directly enhance neutrophil expression of adhesion protein CD11b, CD16 and prime neutrophil oxidative burst. The exceedingly low level of IL8 and TNFα found in this study suggests that other factors may play a more important role in neutrophil priming and activation.