A series of (o- and p-nitrobenzyloxycarbonyl)-5-fluorouracil derivatives were synthesized by reacting o- or p-nitrobenzyl chloroformate with 5-fluorouracil in the presence of triethylamine in DMF or Me2SO. The reductive activation of these agents was hypothesized to generate a reactive methide and 5-fluorouracil, two components that are capable of synergistic interaction through complementary inhibition. Measurement of the surviving fractions of EMT6 tumor cells treated with these agents in culture under conditions of hypoxia and aerobiosis resulted in equal cell kill regardless of the state of oxygenation. One of the synthesized agents, 3-(p-nitrobenzyloxycarbonyl)-5-fluorouracil (4), appeared to be superior to 5-fluorouracil in prolonging the survival time of mice bearing intraperitoneal implants of the P388 leukemia and Sarcoma 180.