The requirements for active-site binding of thymidylate synthase from three sources, Lactobacillus casei, murine leukemia L1210, and human lymphoblast (Molt/4F), were investigated by analyzing the binding of a series of 5-(p-substituted phenyl)-2'-deoxyuridylates (N1-substituted 5-aryl-2, 4-dioxopyrimidines) to the enzyme. Multiple regression analysis revealed that an increase in electron density of the heterocyclic ring and hydrophobic substituents enhance affinity. Correlations of biological results with spectral data indicated that higher electron densities at the oxygen atoms are responsible for increase in binding. These results support the presence of both a cationic binding site and a hydrophobic region. In addition, the results revealed an unusual reversal of electronic requirements for binding and catalysis. The formation of the binary complex is enhanced by electron-donating substituents, while the initial catalytic reaction, formation of the covalent ternary complex, is promoted and stabilized by electron-withdrawing substituents.