The novel fluoropyrimidine, 5'-deoxy-5-fluorouridine (5'-dFUDR) has strong tumor-inhibiting effect without severe cytotoxic effects on the normal cells. This pro-drug is metabolized by the enzyme pyrimidine nucleoside phosphorylase (PN'ase) to the active form 5-fluorouracil. Comparative determinations of pyrimidine nucleoside phosphorylase activity in tissue extracts revealed a significantly higher activity in the human gastrointestinal cancer tissues (6.84 +/- 0.70 nmol/mg protein) than in normal tissues from the same organ (2.37 +/- 0.21 nmol/min/mg protein).