The purpose of this study was to document the effects of TBZ on the early stages of T cell maturation. The method utilized to quantify presumptive thymocyte progenitors in the bone marrow used the enzyme terminal deoxynucleotidyl transferase (TdT) as a marker. Shifts in thymocyte populations were determined by quantitative planimetry, immunofluorescence for TdT and tritiated thymidine incorporation. When given alone, TBZ produced a marked increase in the number of large, mitotically active lymphoblasts in the thymus cortex. When given with DNFB, a thymus-dependent neoantigen, TBZ produced a significant increase in TdT-positive bone marrow cells. In a companion study, TBZ + DNFB were found to stimulate T cells in lymph node and spleen. Hence, the results indicate that under appropriate conditions, TBZ can stimulate all stages of T cell differentiation in mice.