Background and Objectives: Cytokines in platelet concentrates contribute to febrile transfusion reactions. Activated monocytes are a major source of inflammatory cytokines, however the role of monocytes in cytokine production in platelet concentrates has not been clarified. This study undertook to quantitate monocytes, determine whether monocyte activation occurs and identify an association with IL-6 and IL-Iβ concentrations in platelet concentrates. Materials and Methods: 17 platelet concentrates were analysed for total leucocyte and monocyte counts, CD14 and CD16 monocyte-associated antigen expression and IL-1β and IL-6 measurements on days 1, 2, 3, 4 and 5. Results: Monocytes in all platelet concentrates expressed increased levels of CD14 and CD16 from day 1 of storage. 10/17 platelet concentrates had elevated IL-6 levels by day 3. Platelet concentrates with IL-6 levels above 15 pg/ml on day 5 had monocyte counts between 0.14 and 15.6 x 106/unit on day 1, while those with IL-6 levels below 15 pg/ml had low monocyte counts of < 0.01 to 1.2 x 106/unit on day 1. Conclusion: Monocytes present in platelet concentrates exhibit features of activation. Monocyte activation is present following the preparation of platelet concentrates, implicating the manufacturing process in its development. Increased IL-6 and IL-Iβ levels during platelet concentrate storage are commonly associated with a higher monocyte count. However, no direct association could be identified between the extent of monocyte activation and the level of cytokine release.