Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase that catalyses the polymerization of deoxynucleotides at the 3' hydroxyl ends of oligo or polydeoxynucleotide initiators and functions without a template. It is expressed in primitive T and B lympho- cytes of the thymus and bone marrow, and appears to have been highly activated in certain leukaemias. This antibody will help identify TdT in normal and uncharacterized tissues.