Nucleoside analogues with modified sugar moieties have been examined for their substrate/inhibitor specificities towards highly purified deoxycytidine kinase (dCK) and thymidine kinases (tetrameric high-affinity form of TK1, and TK2) from human leukemic spleen. In particular, the analogues included the mono- and di-O'-methyl derivatives of dC, dU and dA, syntheses of which are described. In general, purine nucleosides with modified sugar rings were feebler substrates than the corresponding cytosine analogues. Sugar-modified analogues of dU were also relatively poor substrates of TK1 and TK2, but were reasonably good inhibitors, with generally lower Ki values vs TK2 than TK1. An excellent discriminator between TK1 and TK2 was 3'-hexanoylamino-2',3'-dideoxythymidine, with a Ki of approximately 600 microM for TK1 and approximately 0.1 microM for TK2. 3'-OMe-dC was a superior inhibitor of dCK to its 5'-O-methyl congener, consistent with possible participation of the oxygen of the (3')-OH or (3')-OMe as proton acceptor in hydrogen bonding with the enzyme. Surprisingly alpha-dT was a good substrate of both TK1 and TK2, with Ki values of 120 and 30 microM for TK1 and TK2, respectively; and a 3'-branched alpha-L-deoxycytidine analogue proved to be as good a substrate as its alpha-D-counterpart. Several 5'-substituted analogues of dC were good non-substrate inhibitors of dCK and, to a lesser extent, of TK2. Finally, some ribonucleosides are substrates of the foregoing enzymes; in particular C is a good substrate of dCK, and 2'-OMe-C is an even better substrate than dC.