The antitumor activity of a glucuronide of 5-fluorouracil, methyl (5-fluoro-1H-2-oxo-4-pyrimidinyl beta-D-glucopyranosid) uronate (FU-O-G), which is a 5-fluorouracil (5-FU) derivative with remarkably low toxicity, was studied. The antitumor activity of this compound was superior to those of 5-FU and 1-(2-tetrahydrofuryl)-5-fluorouracil (Tegafur, Ftorafur) in the treatment of transplantable tumors, not only 5-FU-sensitive tumors such as adenocarcinoma 755, lymphosarcoma LS-1, and plasmacytoma X5563, but also 5-FU-resistant tumors such as Lewis lung carcinoma and Walker carcinosarcoma 256. Furthermore, in the treatment of Lewis lung carcinoma, which responds poorly to 5-FU and Ftorafur, daily administration of FU-O-G at a dose of 400 mg/kg for 30 days produced a 92% increase in life span without marked loss of body weight, though short-term administration (such as 5 days) was barely effective. Thus, it appears that FU-O-G is an antitumor agent suitable for long-term administration. These findings correspond with the results of an enzymological study which showed selective activation of FU-O-G by beta-glucosidase in tumor cells and indicated that the compound is a marked form of 5-FU.