Oncolytic vaccinia virus (VV) has shown promise for the delivery of gene therapy and has the potential to be delivered systemically to treat metastatic cancer. VV is known to induce a strong host immune response, which requires immunocompetent models for evaluation. We investigated the systemic delivery of oncolytic vaccinia virus to murine tumours in immunocompetent mice using tumour-targeted oncolytic vaccinia virus and found that VV was unable to efficiently infect tumours due to clearance, mostly by splenic macrophages. Depletion of macrophages using clodronate liposomes (CL) facilitated the systemic delivery of vaccinia virus to subcutaneous, established murine tumours. However CL non-selectively deplete macrophages and potentially diminish any beneficial macrocytic activity in the tumour microenvironment unrelated to viral clearance. Consequently, a more appropriate agent is needed.Macrophages recognise and ingest pathogenic microorganisms through phagocytosis, a process for which several lines of evidence have highlighted an important role for phosphatidylinositol 3-kinases. Thus, PI3K inhibitors could potentially enhance systemic delivery of VV. Eight mammalian isoforms of PI3K exist, but the specific isoforms involved in macrophage phagocytosis of VV have not been elucidated. In this study, we, for the first time, demonstrated that PI3K delta is a major player affecting monocytes to clear vaccine virus. IC87114 (p110 delta inhibitor) is effective at reducing uptake of VV by macrophages. Subsequently, it was confirmed that IC87114 affects attachment of the virus to macrophages but plays no role in internalisation. Furthermore IC87114 is neither directly toxic to a panel of cancer cells nor oncolytically additive nor synergetic when combined with VV in vitro. Biodistribution studies have established that IC87114 combined with VV results in statistically more virus detected in tumours compared to the VV alone treated groups with similar limited off target effects. Finally three different efficacy studies have demonstrated statistically significant superior tumour response in IC87114 group. In conclusion, p110 delta blockade is an effective strategy for enhancing systemic deliver of VV in a pre-clinical model and could be a useful adjuvant in VV clinical trials