Platelets are highly active cells of the blood system, responsible for primary haemostasis and arterial thrombosis. To achieve this they have tightly regulated adhesive properties, secretory mechanisms and a highly dynamic cytoskeleton. These functions may be activated by a multitude of receptors for agonists ranging from small soluble molecules to large macromolecular structures such as collagen. In order to transduce signals to functional events, platelets express a wide array of intracellular signalling components including a wide range of protein kinases. Importantly amongst them platelets express members of the protein kinase C family, and in human platelets the most prominent isoforms to be expressed are PKCα, PKCβ, PKCδ and PKCθ (Crosby & Poole, 2002). Multiple pharmacological studies have demonstrated a critical role for the PKC family in regulating all major functional activities in platelets, but we are only now beginning to elucidate the roles played by individual isoforms in regulating these activities. We have used combinations of pharmacological and genetic approaches to dissect out roles of individual PKC isoforms in regulating the various functional activities carried out in platelets in response to a variety of agonists. In particular, roles of the classical isoforms, PKCα and PKCβ, and a novel isoform, PKCδ, will be presented in this abstract. Importantly we reveal that the classical isoforms PKCα & β play both redundant and non-redundant roles. We show that PKCα is critical and required for dense granule secretion, with little contribution from PKCβ, whereas for aggregation there is redundancy of function between the two isoforms, where absence of one isoform may be compensated by presence of the other. In contrast to the positive contributions made by the classical isoforms, the novel isoform PKCδ plays a largely negative signalling role. This is mediated through a physical and functional interaction of PKCδ with VASP, leading to inhibition of filopodia formation, such that in the absence of PKCδ filopodia formation is no longer transient but sustained. VASP is essential for mediating signalling by PKCδ, since the effects of the PKCδ-selective inhibitor rottlerin on filopodia formation, actin polymerisation and platelet aggregation are ablated in VASP-/- platelets. The work therefore reveals a new pathway for regulation of actin and filopodia, and thereby platelet aggregation, and shows PKCδ to be a major negative regulator of these events (Pula et al. 2006).