Terminal deoxynucleotidyl transferase (TdT) represents a useful marker for the diagnosis of acute lymphoblastic leukemia and lymphoblastic lymphoma. It is usually assayed in fresh tissue or cell suspensions by enzyme analysis, immunofluorescence, or immunoperoxidase. We have obtained satisfactory staining for TdT in routinely processed paraffin sections using a recently described antigen retrieval pretreatment based on microwave oven heating of the sections and a polyclonal rabbit anti-TdT serum. With this technique, we assayed paraffin sections from lymph nodes of 91 patients with a variety of malignant lymphomas, including 35 cases of lymphoblastic lymphoma. The specificity of this immunoperoxidase method of TdT analysis was confirmed by comparing the results obtained with conventional TdT analysis by indirect immunofluorescence. Neoplastic cells from 33 of the 35 (94%) patients with lymphoblastic lymphoma were TdT positive with both techniques. All remaining 50 cases of other subtypes of malignant lymphoma were TdT negative. Specificity was further confirmed by demonstrating TdT-positive blasts in paraffin sections of bone marrow biopsies from 44 patients with TdT-positive (by indirect immunofluorescence) acute leukemias and in paraffin sections of extramedullary leukemic infiltrates from eight patients with relapsed acute lymphoblastic leukemia. Other control material studied included eight lymph nodes with Hodgkin's disease, four normal spleen, seven reactive lymph nodes, and 25 nonneoplastic bone marrow samples, all of which gave the expected results, i.e., rare scattered TdT-positive cells in bone marrow and lymph nodes, absence of staining in Hodgkin's disease malignant cells and spleen. Our results confirm that TdT is a specific marker for the diagnosis and classification of lymphoblastic lymphoma.(ABSTRACT TRUNCATED AT 250 WORDS)