Oncolytic cancer therapy using herpes simplex viruses (HSV) that have direct tumoricidal effects and cancer immunotherapy using the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) have each been effective in preclinical testing. NV1034 is a multi-mutated oncolytic HSV carrying the gene for murine GM-CSF that attempts to combine these two anticancer strategies. The purpose of this study was to compare NV1034 to NV1023, the parent HSV mutants lacking GM-CSF, in order to determine if such combined oncolytic and immunotherapy using a single vector has advantages over oncolytic therapy alone. In vitro, expression GM-CSF did not alter the infectivity, in vitro cytotoxicity, or replication of NV1034 compared to the non-cytokine secreting control. Tumors infected with NV1034 produced GM-CSF in picogram quantities. In vivo efficacy of the viruses against murine colorectal carcinoma CT26 and murine hepatoma Hepa l-6 was then tested in subcutaneous tumors in syngeneic Balb/c and C57 L/J mice respectively. In these immune competent models, NV1034 or NV1023 each demonstrated potent antitumor activity. Treatment with NV1034 had significantly better antitumor effect compared to treatment with NV1023. Furthermore, in mice depleted of CD4+ and CD8+ T-lymphocytes, there was no difference in the antitumor efficacy of these viruses. Viral vectors combining oncolytic and immunotherapy are promising agents in treatment of colorectal carcinoma and hepatoma.