An immunoconjugate consisting of the anti-tumor cytokine interleukin-12 (IL-12) fused to the tumor-targeting antibody BC1 with potential immunostimulatory and antineoplastic activities. The antibody moiety of huBC1-huIL12 fusion protein AS1409 binds to the human fibronectin splice variant ED-B, delivering IL-12 directly to the tumor vasculature; tumor vasculature-targeted IL-12 initiates localized immune cascade responses and exhibits cytotoxic and anti-angiogenic activity while minimizing the systemic side effects of IL-12. The human fibronectin splice variant ED-B is over-expressed in the extracellular matrix and blood vessels of tumor tissues.