To establish a novel drug delivery system for 5-fluorouracil (5FU), we have developed a system in which the low-molecular-weight prodrugs of 5FU bound to endogenous serum proteins, thus circulating like those proteins. Subsequently, the prodrugs were slowly hydrolyzed to generate active 5FU in the bloodstream. To examine the therapeutic effect of these prodrugs, we injected them into BALB/c mice previously implanted subcutaneously with Meth A sarcoma. Among the prodrugs, 1-(N-4-chlorophenyl-N-methylcarbamoyl)-5-fluorouracil (5FU-1pCPMC) was effective in reducing tumors and prolonging survival time. The non-hydrolyzable compound, 1-(4-chlorobenzyl)-5-fluorouracil, did not show any therapeutic effect, suggesting that the therapeutic efficacy of 5FU-1pCPMC is due to the sustained release of 5FU from the serum protein-prodrug complex. The data shown here may create a new field in drug delivery system technology.