SUMMARY A simplified procedure for purification of terminal deoxy- nucleotidyl transferase is presented. The enzyme is shown to be homogeneous by equilibrium centrifugation and gel electrophoresis. The molecular weight of the enzyme is 32,360, with a V measured as 0.65 cc g-r. The homogeneous enzyme can be dissociated into two subunits, CY and @, by sodium dodecyl sulfate. Subunit molecular weights, esti- mated from gel electrophoresis, are 01 = 8,000 and fl = 26,500. The procedure for partial purification of a terminal deoxy- nucleotidyl transferase from the soluble protein fraction of calf thymus gland reported earlier from this laboratory (1) produces an enzyme that is free of degradative activity and has a useful specific activity. In the earlier work the terminal transferase was fractionated as a contaminant of DNA polymerase and the assay used for terminal transferase did not permit its detection in crude extracts. This report presents some attempts to estimate the amount of terminal trasferanse activity in crude extracts. These experi- ments indicate that our earlier purification procedure, although not specifically oriented toward terminal transferase purification, does provide a major fraction of this activity. We have therefore continued the fractionation and have devised a simple procedure for obtaining a terminal transferase preparation that is homo- geneous by equilibrium centrifugation and gel electrophoresis. Preparations of the homogeneous enzyme have a molecular weight of 32,360 and can be dissociated by sodium dodecyl sulfate into two subunits having molecular weights of 26,500 (/3 chain) and 8,000 (a chain).