Thymidylate synthase (TS) is a rate-limiting enzyme in de novo DNA biosynthesis and also a primary target for 5-fluorouracil (5-FU)-based chemotherapy. To estimate the con-elation between the expression of TS in the tumor, clinical response and prognosis in cancer patients treated with 5-FU, we have prepared recombinant human TS (rhTS) protein by culturing of E. coli transfected with the plasmid (pGEX-2TH) encoding the nucleotide sequence of hTScDNA and then obtained a highly purified polyclonal antibody against rhTS protein. Using this anti-TS antibody, it was revealed that the content of TS protein, as determined by Western blot analysis, correlated with the enzyme activity (gamma=0.973) and cytotoxicity of 5-FU, expressed as IC50 value (gamma=0.954) against human colon tumor cells, sensitive to and with acquired-resistance to 5-fluoropyrimidines and other cancer cells. On immunochemical evaluation, it was also confirmed that the tumor cells overexpressing TS proteins were strongly stained by the polyclonal antibody when compared to the cell lines expressing TS to lower extent both in vitro and in vivo conditions. These results indicate that this purified polyclonal antibody to rhTS is applicable to prospective and retrospective clinical studies on immunochemical TS expression in various tumors as a prognostic factor and 5-FU response-predicting parameter.