The antitumor activity of the fluorinated pyrimidines 5-fluorouracil (FUra), 5-fluorouridine (FUrd), and 5-fluoro-2'-deoxyuridine (FdUrd) against P388 leukemia was markedly potentiated by the addition of guanosine (Guo), resulting in therapeutic synergism. Any combination of FUra at 1-20 mg/kg, FUrd at 0.3-1 mg/kg, or FdUrd at 1-100 mg/kg with Guo at 100 mg/kg significantly potentiated the activity of FUra, FUrd, or FdUrd, respectively. The potentiation of these fluorinated pyrimidines by guanosine was abolished by the simultaneous administration of cytidine or uridine, but not of thymidine. In particular, cytidine was the strongest inhibitor of antitumor activity of these fluorinated pyrimidines, alone and in combination with guanosine. To obtain more effective treatment with the combination of various fluorinated pyrimidines and Guo, the influence of the molar ratios of Guo to the fluorinated pyrimidines on the antitumor activity against P388 leukemia was investigated. The increase in life-span became more pronounced with increasing molar ratios. The optimal molar ratios of Guo/FUra, Guo/FUrd, and Guo/FdUrd were more than 5, 100, and 5, respectively.