This paper presents improved synthetic methods for the modification of 2'-deoxyuridine-5'-monophosphate and its 5-fluoro derivative, using trimethylphosphate in aqueous medium at pH 10. These modifications include methylation of the pyrimidine ring N(3) and/or esterification of the phosphate group. The 5'-methyl ester of dUMP was neither a substrate nor an inhibitor of Ehrlich ascites carcinoma thymidylate synthetase. By contrast, the corresponding methyl ester of FdUMP was a tight-binding inhibitor of the enzyme from L1210, Ehrlich ascites carcinoma and CCRF-CEM cells. 3-Methyl-dUMP, fixed in the 4-keto form, exhibited only very weak substrate activity with the Ehrlich ascites carcinoma enzyme. The dUMP analogues 5-ethyl-dUMP and 5-propyl-dUMP were found to be competitive inhibitors of thymidylate synthetase from L1210, Ehrlich ascites carcinoma and HeLa cells, the former being the more potent inhibitor. Both analogues were shown to bind cooperatively to each of the mouse tumour enzymes. Two molecules of inhibitor interacted with a single enzyme molecule, reflected by the parabolic character of the replots of the slope versus inhibitor concentration. The parent dTMP was a stronger inhibitor of the mouse tumour enzymes than its higher alkyl homologues.