L-nucleoside analogues such as lamivudine are active for treating viral infections. Like D-nucleosides, the biological activity of the L-enantiomers requires their stepwise phosphorylation by cellular or viral kinases to give the triphosphate. The enantioselectivity of NMP kinases has not been thoroughly studied, unlike that of deoxyribonucleoside kinases. We have therefore investigated the capacity of L-enantiomers of some natural (d)NMP to act as substrates for the recombinant forms of human uridylate-cytidylate kinase, thymidylate kinase and adenylate kinases 1 and 2. Both cytosolic and mitochondrial adenylate kinases were strictly enantioselective, as they phosphorylated only D-(d)AMP. L-dTMP was a substrate for thymidylate kinase, but with an efficiency 150-fold less than D-dTMP. Both L-dUMP and L-(d)CMP were phosphorylated by UMP-CMP kinase although much less efficiently than their natural counterparts. The stereopreference was conserved with the 2'-azido derivatives of dUMP and dUMP while, unexpectedly, the 2'-azido-D-dCMP was a 4-fold better substrate for UMP-CMP kinase than was CMP. Docking simulations showed that the small differences in the binding of D-(d)NMP to their respective kinases could account for the differences in interactions of the L-isomers with the enzymes. This in vitro information was then used to develop the in vivo activation pathway for L-dT.