Antibody to purified terminal deoxynucleotidyl transferase (nucleosidetriphosphate : DNA deoxy-nucleotidylexotransferase, E.C. 2.7.7.31) from calf thymus was prepared in rabbits using terminal deoxynucleotidyl transferase crosslinked to bovine serum albumin. These antibodies, partially purified by 60% ammonium sulfate precipitation and Sephadex G-200 column chromatography, produced one precipitation band with calf thymus terminal deoxynucleotidyl transferase on immunodiffusion. This antibody preparation also inhibited the in vitro activity of terminal deoxynucleotidyl transferase from calf thymus, acute leukemic lymphoblasts and Molt-4 cells but not that of DNA polymerases alpha, beta and psi from these cells