Terminal-deoxynucleotidyl-transferase (TdT) activity determinations, using the indirect immunofluorescence technique, were performed on 28 consecutive leukemia patients. Fifteen patients with acute lymphoblastic leukemia had 38 bone marrow smears examined. TdT-positive cells were a good indicator of disease activity in acute lymphoblastic leukemia in that numbers were increased at the time of diagnosis, normal during remission, and increased during relapse. Ten patients with acute myeloid leukemias had 42 smears studied. The number of TdT-positive cells did not reflect the activity of the disease, except for two patients with acute myelomonocytic leukemia in which TdT-positive cells were mildly increased during relapse. Three patients with chronic myelogenous leukemia had eight smears studied. The number of positive cells seemed meaningful only in following the course of lymphoid blast crisis. It would therefore appear that TdT determinations seem to be of value in lymphoblastic leukemia, but of limited value in myeloid leukemias.