Moderately high levels of activity of the enzyme terminal deoxyribonucleotidyl transferase (TdT) were found in the leukemic cells of a patient with acute lymphyocytic leukemia. The proliferating cells were B lymphocytes bearing IgG antibody, and the disease was associated with an IgG monoclonal spike and a mediastinal mass. The observations in this case suggest that TdT is related more to the immaturity and proliferation of certain lymphoid stem cells than to their progress toward B- or T-cell differentiation.