Structural changes in the macromolecular targets of pharmacological agents can result in alterations in the efficacy of these agents. In previous studies Berger et al. (1) identified a variant structural form of thymidylate synthase (TS) that is associated with relative resistance to 5-fluoro-2'-deoxyuridine, in a human colonic tumor cell line. They observed that expression of the variant TS, which differs from the normal form by a tyrosine to histidine substitution at residue 33, confers a 4-fold level of drug resistance in mammalian cells, as well as in bacteria. Now we report on the use of RT-PCR techniques to see if that variant TS form could be present in human samples from patients who underwent surgery for primary colorectal cancer and been previously untreated and to try to find relationships between that hypothetical variant TS form and the 5-Fluorouracil treatment. The possible role of Tyr-33 in 5-fluoropyrimidine-mediated inhibition of TS is discussed.