Interferon-gamma (IFN-gamma) has been used in the experimental treatment of cancer with limited success. Despite direct cytotoxic effects on tumor cells and the ability to stimulate the antitumor activities of a variety of effector cells, IFN-gamma has not been found to produce impressive therapeutic responses partly because of inadequate sustained intratumoral concentrations and systemic toxicity. To overcome these obstacles, we have developed an antibody/murine IFN-gamma fusion protein (chTNT-3/muIFN-gamma), which utilizes the tumor necrosis therapy antibody, chTNT-3, to target murine IFN-gamma to necrotic regions of solid tumors implanted in immunocompetent BALB/c mice. The genetically engineered fusion protein was expressed in NS0 cells using the Glutamine Synthetase Gene Amplification Expression System. After purification, the fusion protein demonstrated both antigen targeting and cytokine activities as assessed by in vitro assays which, when compared to recombinant free IFN-gamma, demonstrated approximately 40-45% biologic activity by two separate assay determinations. Pharmacokinetic and biodistribution studies in mice demonstrated a relatively long whole body half-life of 32 h in vivo and significant intratumoral accretion, respectively. Most importantly, immunotherapeutic studies in the MAD109 syngeneic murine carcinoma of the lung demonstrated significant intratumoral infiltration by leukocytes, primarily by macrophages and CD4(-) CD8(-) Thy-1.2(+) lymphocytes. Additionally, intravenous administration of the fusion protein significantly decreased the number of metastatic foci in an experimental model of pulmonary metastasis without causing any observable toxicity. These studies demonstrate that chTNT3/muIFN-gamma can safely target syngeneic tumor models as part of a promising strategy for the targeted immunotherapy of solid tumors.