Introduction: Using in vitro and in vivo models of vascular inflammation which resemble aspects of the chronic inflammatory environment in the atherosclerotic artery wall, we have recently described a paradigm where platelets recruited to intact endothelial cell monolayers formed adhesive bridges between the vessel wall and flowing leukocytes. Interestingly, monocytes, the major inflammatory leukocyte population recruited during atherogenesis, were preferentially recruited from flowing blood. In these models, platelet adhesion to endothelial cells preceded interaction with monocytes. However, the formation of platelet-monocyte aggregates in the circulating blood is well documented, as is the propensity for such heterotypic aggregation to increase in patients with cardiovascular and chronic inflammatory disease. It is unknown whether heterotypic aggregation increases the efficiency of monocyte recruitment during vascular inflammation, a phenomenon we will investigate in the near future. The aim of the current study was to determine how efficiently platelet-monocyte aggregates could form in whole blood, in particular monitoring heterotypic aggregation on monocyte subsets in response to different platelet agonists. Methods: Whole blood was treated with the platelet agonists adenosine diphosphate (ADP; 30µM), thrombin receptor activating peptide (TRAP; 100µM), thromboxane mimetic, U46619 (10µM), rhodocytin, (100nM) or collagen related peptide (CRP-XL; 1µg/ml). Blood was placed on a rolling mixer at 37°C and aliquots were periodically taken for analysis. Heterotypic aggregation was assessed by flow cytometry using antibodies against monocyte markers (CD14 and CD16) and against platelet GPIb (CD42b). Statistical significance was determined by ANOVA and Bonfferoni. Results: After treatment with any of the agonists there was a significant increase in heterotypic aggregate formation compared to untreated control for both CD14+CD16- and CD14+CD16+ monocyte subsets, the incidence of aggregate formation did not vary between the subsets. Interestingly, there were significant differences between the rate and efficiency of heterotypic aggregate formation with different agonists, with CRP-XL being the weakest agonist (≈30% of CD14+CD16- monocytes positive for GPIb) and TRAP the most potent (≈65% of CD14+CD16- monocytes positive for GPIb) after 30 minutes of stirring. Conclusions: Platelet-monocyte aggregates form in whole blood in response to a range of platelet agonists, although the efficiency of aggregation varies in an agonist dependent manner. Both monocyte subsets form heterotypic aggregates with equal efficiency. These data imply that the route of platelet activation may be an important arbiter of the efficiency of platelet-monocyte interaction in whole blood and could influence down stream functional outcomes such as monocyte recruitment.