We investigated the antitumor effects of a synthetic lipid A derivative, DT-5461a, in combination with indomethacin in three experimental tumor models (peritoneal carcinomatosis, liver tumor, and lung tumor models) of transplanted colon 26 carcinoma in mice. This carcinoma produces the immunosuppressive prostaglandin E2 (PGE2). Intravenous administration of DT-5461a alone resulted in little or no prolongation of survival time [increase in life span (ILS): -2%-22%]. When indomethacin was given in drinking water a slight or moderate increase in survival time was seen (ILS: 4%-45%). In contrast, the combination of DT-5461a and indomethacin induced an additive increase in life span (ILS: 16% to more than 193%). The strongest antitumor effect of this combined therapy was seen in the peritoneal carcinomatosis model; in this model, plasma PGE2 concentrations were considerably higher than in normal mice, and concentrations were further but transiently increased by DT-5461a administration. Following oral indomethacin administration, these elevated PGE2 concentrations were reduced to the level in untreated normal mice. Furthermore, intratumoral tumor necrosis factor (TNF) activity in the group receiving the combined therapy was significantly higher than that in the DT-5461a-treated group. No TNF production was induced by the administration of indomethacin alone. These results suggest that the antitumor effect of DT-5461a can be enhanced by combination with indomethacin, and that the inhibition of PGE2 production may have a role in this antitumor effect.