Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase of 58KD located in the cell nucleus which catalyses the polymerisation of deoxynucleotides at the 3= hydroxyl ends of oligo or polydeoxynucleotide initiators and functions without a template. TdT is expressed in primitive T and B lymphocytes of the normal thymus and bone marrow. The identification of TdT-positive cell populations in primary and secondary lymphoid organs during maturation of the immune system is one area of interest but it is the occurrence of high levels of enzyme activity in white blood cells and marrow in certain leukaemias which is of particular interest. Early and precise diagnosis of lymphoblastic ly phoma is crucial to direct an appropriate therapeutic regimen.TdT is considered to be a highly specific marker for the diagnosis and classification of acute lymphoblastic lymphoma/leukaemias.The determination of TdT expression is most valuable when it is  difficult to differentiate histologically between lymphoblastic lymphoma and Burkitt=s lymphoma.This antibody will be of use in the differentiation of lymphoblastic lymphoma from other lymphomas.