Twenty-two chemotherapy-resistant patients with liver metastases received 46 courses of recombinant human tumor necrosis factor (rhTNF) administered by 5-day continuous infusion through percutaneously inserted hepatic arterial catheters. The maximum tolerated daily dose of rhTNF was 150 micrograms/m2. This is six times the maximum tolerated daily dose of rhTNF that could be given systemically (intravenous) on the same schedule. The dose-limiting toxicity resulted in severe, although transient, hypophosphatemia (less than 1.0 mg/dl) associated with myocardial dysfunction. Objective tumor response (partial tumor response or greater) was observed in 2 of 14 patients (14%) with colorectal cancer and lasted as long as 3 months. Three additional minor responses occurred among these patients with colorectal cancer. Plasma carcinoembryonic antigen levels also decreased significantly (greater than 25%) in 7 of the 14 (50%) patients with colorectal cancer. Regional biologic therapy with rhTNF as a sole modality has definite antitumor activity in colorectal cancer metastatic to the liver and warrants additional study in previously untreated patients.