Herpes simplex viruses type 1 (HSV-1) that lack the γ134.5 gene are unable to replicate in the central nervous system (CNS), but maintain replication competence in actively dividing tumors. To determine if antitumor therapy by M002, a γ134.5- HSV that expresses interleukin-12 (IL-12), could be augmented by combinatorial therapy with another γ134.5-deleted HSV-1 engineered to express the chemokine CCL2, Neuro-2a tumors were established subcutaneously in the syngeneic A/J mouse strain. Tumors received multiple injections intratumorally either of saline, the parent, non-cytokine-expressing virus R3659, M002, M010 (γ134. 5- HSV expressing CCL2), or a combination of M002 and M010. Efficacies were evaluated by monitoring inhibition of tumor growth over time. Results demonstrated the following: (1) inhibition of tumor growth was most pronounced in tumors treated with a combination of M002 and M010; (2) enhanced tumor growth inhibition for the combinatorial treatment group was statistically significant compared to either M002 or M010 alone; and (3) the variability between slopes of the tumor growth rates within an individual treatment group appeared to be virus-dependent, and was reproducible between experiments. Our results demonstrate that combinatorial cytokine/chemokine γ134. 5- HSV therapies can provide superior antitumor effects in experimental tumors as a model for malignancies arising in the brain. © 2005 Nature Publishing Group All rights reserved.