Homogeneous human deoxycytidine kinase was purified in one step from a variety of spontaneous human leukemic cells (T-ALL, B-ALL, B-CLL, AML, CML), and from cultured T-lymphoblast cells (MOLT-4) using the newly developed affinity medium, $dCp_4$-Sepharose. Starting with an ammonium sulfate fraction, purification was achieved in one step with the kinase being eluted from a column by the end product inhibitor, dCTP. The purified deoxycytidine kinase from T-ALL cells phosphorylated deoxyadenosine and deoxyguanosine, as well as deoxycytidine. The enzyme purified from T-ALL and B-CLL cells yielded one major band with a molecular weight of 52 kDa determined by SDS-polyacrylamide gel electrophoresis. AML and CML cells yielded one 52 kDa band and an extra band of 30 kDa molecular weight. On the other hand, B-ALL and MOLT-4 cells showed a low molecular weight band of 30 kDa only. However, the electrophoretic mobilities of enzymatic activity in 12% non-denaturing gels were identical for the dCyd kinase from all different kinds of leukemic cell lines, except that the B-ALL, B-CLL, and MOLT-4 cell preparations had an extra minor peak, all at the same position. dAdo and dCyd phosphorylating activities comigrated indicating that these activities are all associated with the same protein. Two new methods, a disk implantation method and a nitrocellulose powder method were used with a small amount of enzyme protein to raise polyclonal antibodies against dCyd kinase purified from T-ALL cells.