Systemic treatment with tumor necrosis factor (TNF) is associated with side-effects, limiting its clinical use in the treatment of malignancies. To investigate the feasibility of other routes of administration experimental and clinical studies were started to establish the toxicity and antitumor activity of TNF after intratumoral (i.t.) injection. In a rat model for colon adenocarcinoma, tumor fragments, implanted subcutaneously or under the hepatic capsule, were treated with TNF injected i.v. or i.t. A dosage of 40 micrograms/kg was lethal when given i.v., but not i.t. Injection of TNF (40 micrograms/kg) directly into the tumor resulted in inhibition of tumor growth in the subcutaneous as well as subhepatic tumor model. A phase I study was started in patients with advanced malignancies to determine the toxicity of TNF injected into liver metastases. Injection of TNF into liver metastases was accomplished by ultrasonography. A 50 microgram-dose escalating schedule (3 patients/dosage) was chosen, starting at a dose of 100 micrograms TNF/injection. Up to now, 12 patients have been treated, the highest dosage of TNF injected being 250 micrograms. Chills, fever, nausea and vomiting were the main side-effects. No significant changes were found in circulatory, hematologic, renal and liver parameters. In summary, i.t. administration of TNF is associated with antitumor efficacy in experimental models and well-tolerated in man. The antitumor efficacy of TNF i.t. in man awaits evaluation in a phase II study.