Nucleoside monophosphate kinase (EC 22.214.171.124) catalyzes the phosphorylation of various nucleoside monophosphates to their corresponding diphosphates. We investigated whether 5-methyl-2'-deoxycytidine 5'-monophosphate (5MedCMP) could serve as a substrate for the enzyme isolated from bovine liver. Although the substrate activity of UMP, CMP and dCMP was readily demonstrable, no activity was recorded with 5MedCMP as the candidate substrate. Moreover, 5MedCMP did not affect the active site of the enzyme, since no inhibition in the phosphorylation of UMP was recorded in the presence of 5MedCMP. This metabolic step appears to be the key phase where the incorporation of exogenous 5-methylcytosine (5MeCyt) into DNA is prevented. Hence, very little or no salvage of DNA 5MeCyt can be expected.