We investigated the efficacy of interferon (IFN)-beta therapy against colon cancer using a novel approach mediated by a lyophilized preparation of High 5 (H5) insect cells transduced with a recombinant baculovirus encoding murine IFN-beta (H5BVIFN-beta). The orthotopic model of CT-26 murine colon cancer in syngeneic BALB/c mice was used in the study, and H5BVIFN-beta was intratumorally delivered. Two injections of H5BVIFN-beta (on days 14 and 21 after tumor cell implantation), but not lyophilized H5 cells (control), significantly reduced the size of cecal tumors and the number of liver metastases. Immunohistochemical analysis revealed that cecal tumors injected with saline or H5 contained many proliferating cells (PCNA+) and few apoptotic cells (TUNEL+). In sharp contrast, H5BVIFN-beta-treated tumors contained fewer PCNA+ cells and significantly more TUNEL+ cells. The H5BVIFN-beta-treated tumors were infiltrated by a large number of CD8+ and F4/80+ cells and expressed a high level of inducible nitric oxide synthase (iNOS). Immunofluorescent double staining technique demonstrated a significant increase of apoptotic endothelial cells (CD-31+/TUNEL+) in tumors treated with H5BVIFN-beta. In conclusion, the data show that intralesional injections of H5BVIFN-beta can suppress the progressive growth of established orthotopic tumors of colon cancer cells and occult liver metastasis. The therapeutic effects directly correlate with destruction of tumor vasculature.