A novel biopharmaceutical, consisting of the F8 monoclonal antibody (specific to a splice isoform of fibronectin) simultaneously fused to both tumor necrosis factor and interleukin-2, was found to react with the majority of solid tumors and hematological malignancies in mouse and man, but not with healthy adult tissues. The product selectively localized to neoplastic lesions in vivo, as evidenced by quantitative biodistribution studies using radioiodinated protein preparations. When the potency of the cytokine payloads was matched by a single-point mutation, the resulting fusion protein (IL2-F8-TNFmut) eradicated soft-tissue sarcomas in immunocompetent mice, which did not respond to individual antibody-cytokine fusion proteins or by standard doxorubicin treatment. Durable complete responses were also observed in mice bearing CT26, C1498 and F9 tumors. The simultaneous delivery of multiple pro-inflammatory payloads to the cancer site conferred protective immunity against subsequent tumor challenges. A fully-human homologue of IL2-F8-TNFmut, which retained a selectivity similar to its murine counterpart when tested on human material, may open new clinical applications for the immunotherapy of cancer.