Terminal deoxynucleotidyltransferase has been purified from lymphoblasts of leukemic patients. The enzyme has a molecular weight of approximately 62,000 as determined by gel filtration and nondenaturing gel electrophoresis and is not dissociated into subunits by sodium dodecyl sulfate. In contrast, the terminal transferase enzyme from calf thymus has a molecular weight of 42,000 as determined by gel filtration, and is dissociated into 2 subunits of Mr 30,000 and 8,000 by sodium dodecyl sulfate. The enzyme has an isoelectric point of 8.2 and kinetic characteristics which are similar to those of calf thymus terminal transferase. The apparent Km for purine nucleotide polymerization at saturating initiator concentration with Mg2+ is 0.2 mM and with Mn2+ is 0.05 mM. Like calf terminal transferase, the reaction velocity is higher in the presence of Mg2+ than Mn2+. ATP inhibits the reaction catalyzed by terminal transferase isolated from human lymphoblasts due to mutual recognition of ATP and dATP by a common site on the enzyme. Preliminary experiments indicate that human terminal transferase may contain a small amount of carbohydrate. This report represents the first purification to near homogeneity of terminal transferase from a tissue source other than calf thymus.