The effects of a synthetic lipid A on tumor necrosis factor (TNF) production and antitumor activity against human pancreatic cancer cells were investigated. Lipid A (10 mg/kg) was injected into normal rats and mice and serum TNF levels were measured. Lipid A-induced inhibition of Molt 4 and MIA paca-2 cells in culture were measured by counting viable cells. Activity of lipid A against transplanted human pancreatic cancer cells (MIA paca-2, Panc-1) was examined by determining tumor volume, necrosis, and survival rate after intraperitoneal injections of lipid A (10 and 20 mg/kg) over 4 weeks. Serum TNF levels increased 80-fold in rats and 100-fold in mice after intravenous lipid A injection. Although specific tumor growth inhibition by lipid A was not observed in vitro, tumor growth was significantly inhibited, and the survival rate was improved in pancreatic cancer cell-transplanted nude mice treated with lipid A compared with controls. Synthetic lipid A induces TNF production and has antitumor activity against transplanted pancreatic cancer cells. Further studies of this lipid A as an agent for pancreatic cancer are warranted.