Terminal deoxynucleotidyl transferase was purified to homogeneity from the blasts of eight patients with leukemia and compared with purified transferase from normal human and calf thymus. In two cases phenylmethanesulfonylfluoride was added during purification to reduce proteolysis. Comparative kinetic analyses of the purified enzymes indicated no differences in catalytic properties. There was substantial variation in the molecular structure of terminal transferase on denaturing polyacrylamide gels: (a) a protein that migrated as a single polypeptide with M(r) = 62,000 was isolated from two patients with acute lymphoblastic leukemia and from MOLT-4 cells; (b) a protein that migrated as a single polypeptide with M(r) = 42,500 was isolated from two patients with acute lymphoblastic leukemia; (c) a protein that migrated as a single polypeptide with M(r) = 42,500 was isolated from two patients with chronic myelogenous leukemia in blast crisis; (d) a protein that migrated as two non-identical subunits of M(r) = 27,000 and 10,000, respectively, was isolated from two additional patients with chronic myelogenous leukemia in blast crisis. The subunit structure of d is characteristic of the homogeneous enzymes purified from human and calf thymus. Neutralizing and precipitating antibodies to terminal transferase from human lymphoblasts and calf thymus have been produced in rabbits and goats. Antisera directed against either human or calf antigens neutralize enzymatic activity and precipitate all forms of human terminal transferase. The multiple human forms give reactions of antigenic identity by immunodiffusion, but differ antigenically from the calf enzyme. The multiple forms of terminal transferase could represent physiological processing, artifactual degradation, or isozymes coded by several genes.