blastic leukemia (24), in cell lines derived from patients with Iymphobbastic leukemia (33), and in cells from a rare patient with acute or chronic myelogenous leukemia in blastic cni

Terminal deoxynucleotidyl transferase activity and cell surface markers were measured in peripheral bymphoid cells from 27 children with acute bymphobbastic leukemia in various phases of their disease. Lymphoblasts from un treated patients had smooth surface ultrastructure but heter ogeneous surface receptors. Greater than 60% of bympho blasts from 4 of 7 untreated patients formed rosettes with sheep red blood cells. Transferase activity was variable, ranging from 8 to 210 units/10@blasts, but it was consist ently elevated at diagnosis and in relapse. Transfemase 1ev els did not correlate with the presence of lymphoblast sun face receptors. During induction therapy transferase activ ity decreased rapidly, but it remained elevated in peripheral lymphoid cells even when blasts were not detectable in the peripheral blood smears. Patients in remission had normal surface receptors and undetectable or minimally elevated bevelsof transferase. Terminal transferase activity may be a sensitive biochemical marker for a primitive cell population and may be important in the evaluation of therapeutic effec tiveness in acute lymphoblastic leukemia.