The antitumor activity of cytostatic 5'-deoxy-5-fluorouridine (5'-dFUrd) depends on its being converted to 5-fluorouracil (5-FUra) by the enzyme thymidine phosphorylase (dThdPase, EC 2.42.4). We prepared mouse anti-human dThdPase monoclonal antibodies to serve as tools for clinical studies with this drug. Partially purified dThdPase obtained form HCT116 human colon cancer cells grown in athymic mice was used as and antigen for the immunization of mice. Six hybridomas were cloned which produced anti-human dThdPase antibodies, as detected by Western blot analysis with human dThdPase. With these antibodies, we developed an ELISA method sensitive enough to measure dThdPase levels, even in tumor tissue samples weighing as little as 10 mg. In addition, one monoclonal antibody was suitable for immunologically staining the enzyme in tumor tissues. Thus, these anti-human dThdPase monoclonal antibodies could be used to measure levels of the enzyme in tumor cells, which is essential for the activation of 5'-dFUrd.