We have developed a flow cytometric assay for the determination of cellular expression of terminal deoxynucleotidyl transferase (TdT) and applied this to the detection of minimal residual T cell acute lymphoblastic leukemia (T-ALL). The flow cytometric assay for TdT demonstrated requisite specificity: TdT was localized to the nucleus, and was detected in MOLT3 T lymphoblasts, clinical T-ALL samples, and normal bone marrow B lymphoid precursors, but in neither the KG1a myeloid leukemia cell line nor normal myeloid cells. Co-expression of TdT and the pan T cell marker CD5 was used to quantify T lymphoblasts. 0.25 +/- 0.13% of normal adult bone marrow CD5+ cells were TdT+; these may represent early T lymphoid precursors. When admixed with normal bone marrow, CD5+TdT+ leukemic cells could be detected above background levels at an added concentration of 0.035% (95% confidence interval 0.028-0.43%). Long term follow-up of a large number of patients will be required to determine the clinical significance of a minimal burden of leukemic cells.