Flow cytometric analysis of terminal deoxynucleotidyl transferase. Terminal deoxynucleotidyl transferase (TdT) is a useful marker for lymphoid precursor cells. In this study, the authors used flow cytometry (FCM) to analyze TdT expression in human hematopoietic malignancies. Cells were fixed in 0.5% formaldehyde, briefly exposed to nonionic detergent, and subsequently labeled with mouse monoclonal anti-TdT antibodies followed by fluorescein-conjugated antimouse IgG and propidium iodide (PI), which was used for the simultaneous analysis of DNA content. Cells from 20 of 22 acute lymphoblastic leukemias (ALL), 4 of 7 mixed lineage leukemias, 2 of 21 acute myeloid and myelomonocytic leukemias, 1 of 2 chronic myeloid leukemias in blast crisis, 1 lymphoblastic lymphoma, and 1 thymoma were TdT positive. Cells from 13 nonlymphoblastic lymphomas, 3 myelodysplastic bone marrows, and peripheral blood mononuclear cells from 29 normal individuals were negative. An excellent correlation was seen between this assay, the conventional slide immunofluorescence technique, and an enzyme immunoassay method. The FCM assay detected as few as 2% blasts in mixing experiments of TdT-containing leukemic cells with normal peripheral lymphocytes. Furthermore, the combined analysis of TdT and DNA allowed the recognition of aneuploid TdT positive cells in four cases of ALL. The high sensitivity, the quantitative information obtained, and the capability of simultaneously analyzing DNA content make this method of TdT analysis more valuable than conventional techniques. 