Abstract Purine nucleoside kinases in human B- and T-lymphoblasts were fractionated by DEAE-cellulose chromatography. Human B-lymphoblast cell extracts showed three peaks of nucleoside kinase activities, adenosine kinase (EC 18.104.22.168), deoxyguanosine kinase and deoxycytidine kinase (EC 22.214.171.124). However, T-lymphoblast cell extracts showed a nucleoside kinase activity which phosphorylates deoxycytidine, deoxyadenosine and deoxyguanosine, similar to deoxycytidine kinase, in addition to the three nucleoside kinases. The K m values of T-lymphoblast-specific nucleoside kinase for deoxyadenosine and deoxyguanosine, 15 and 26 μM, respectively, were smaller than those of deoxycytidine kinase, 150 and 330 μM, respectively. Deoxyadenosine phosphorylation by deoxycytidine kinase was strongly inhibited by dCTP, but the phosphorylation by T-lymphoblast-specific nucleoside kinase was only weakly inhibited by dCTP. Deoxyadenosine phosphyrylating activity in B-lymphoblast extracts was more distinctly inhibited by dCTP than that in T-lymphoblast extracts.