A panel of reagents (OKT1, 3, 4, 6, 8, and 11) detects differentiation antigens expressed exclusively on HuTLA+ T lymphoid cells but absent on bone marrow precursors, such as terminal deoxynucleotidyl transferase- (TdT) positive cells, immature myeloblasts, and other myeloid/erythroid cell types. In the bone marrow no transitional forms could be detected between TdT+ and T cells. The BM T cells showed mostly the suppressor/cytotoxic phenotype (OKT8+) with only a few T cells of inducer type (OKT4+). Thymocyte heterogeneity was also analyzed directly in tissue sections and in double labeling assays in combination with TdT staining, a marker for cortical thymocytes. Many large thymic blasts (in fetal and infant thymus) showed reactivity with OKT11--a pan-T reagent, but had only weak or negligible activity with the other antibodies. Cortical thymocytes reacted strongly with OKT11, 6, 4, and 8, whereas medullary cells reacted with OKT11, 3, 4 (majority) and 8 (minority). Thus, the reactivity with these antibodies is generated in the thymus at various stages of differentiation. In contrast, OKT10 (an anti-"precursor cell" reagent) reacted not only with thymocytes but also with TdT+ BM precursors, myeloblasts, and BM B lymphocytes although it was unreactive with mature peripheral lymphoid and maturing myelo/erythroid cells.