The premise of this project was to engineer a novel viral platform with the capacity to enhance antitumour immunity. To this effect, the N1L gene was disrupted in a Lister strain vaccinia viral backbone that had previously been engineered to be tumour selective (VVL15ΔN1L) and armed with transgenes encoding murine and human versions of GMCSF and IL12. In vitro, they retained potency for infecting, replicating in and killing a panel of murine, Syrian hamster and human cancer cells; all viruses were able to express their transgenes to detectable levels upon infection of every tumour cell line. In comparison to the parental virus (VVL15), VVL15ΔN1L administration into immune competent in vivo tumour models (of pancreatic and lung cancer) led to enhanced intratumour (IT) infiltration of neutrophils as well as markedly elevated circulating numbers of natural killer (NK) cells. VVL15ΔN1L also enhanced the tumour infiltration of CD8+ cells. Functional immunoassays and flow cytometric analysis of T cells provided evidence of enhanced tumour specific adaptive immunity. In comparison to VVL15, IT VVL15ΔN1L significantly reduced the growth of subcutaneously implanted syngeneic pancreatic tumours. This effect was predominantly due to cytotoxic lymphocytes, evidenced by the complete abrogation of efficacy upon repeating the experiment in mice that had been depleted of CD8+ cells. A similar treatment schedule reduced the formation of lung metastases from a primary spontaneously metastasising syngeneic lung cancer model; and translated into prolonged short-term post-operative survival when used as neoadjuvant to surgical resection. Efficacy in this context was contrastingly, due to an elevation in systemic NK cells; concurrent depletion of NK cells (but not CD4+ or CD8+ cells) completely abrogated the survival advantage. The IL12 transgene armed recombinant was the most effective antitumour therapeutic. Its IT administration into pancreatic tumours led to complete tumour eradication in over 80% of tumour bearing mice and was effective in slowing the growth of other aggressive flank tumours. Neoadjuvant administration of VVL15ΔN1L-mIL12 into metastatic lung cancers dramatically prolonged long-term post-surgical survival, with apparent cure of 88% of mice.