By using monoclonal antibodies specific for T lineage surface antigens, neoplastic T cells from 53 children with T cell acute lymphoblastic leukemia and T cell non-Hodgkin's lymphoma were analyzed and assigned to phenotypically defined stages of T cell maturation. Cells were also analyzed for T cell antigen receptor beta-chain gene and immunoglobulin heavy and light chain gene rearrangements. Clonal rearrangements of T cell antigen receptor beta-chain gene and a germ-line configuration of the immunoglobulin genes were found in cells from all cases. The expression of the terminal deoxynucleotidyl transferase (TdT) in leukemic T cells was studied by both qualitative immunofluorescence and quantitative enzyme immunoassay, and the level of TdT expression was correlated with maturational stages. Lymphoblasts classified as prethymic (3 patients) did not express detectable TdT. In contrast, cells from approximately 60% of the patients with early (15 patients) or intermediate (19 patients) thymocytic phenotypes and approximately 40% of the patients with mature thymocytic phenotype (16 patients) were positive for TdT. Thus, in these leukemic clones TdT was randomly expressed and showed no correlation with maturational stage.