Terminal deoxynucleotidyl transferase activity and cell surface markers were measured in peripheral lymphoid cells from 27 children with acute lymphoblastic leukemia in various phases of their disease. Lymphoblasts from untreated patients had smooth surface ultrastructure but heterogeneous surface receptors. Greater than 60% of lymphoblasts from 4 to 7 untreated patients formed rosettes with sheep red blood cells. Transferase activity was variable, ranging from 8 to 210 units/10(8) blasts, but

more »

... t was consistently elevated at diagnosis and in relapse. Transferase levels did not correlate with the presence of lymphoblast surface receptors. During induction therapy transferase activity decreased rapidly, but it remained elevated in peripheral lymphoid cells even when blasts were not detectable in peripheral blood smears. Patients in remission had normal surface receptors and undetectable or minimally elevated levels of transferase. Terminal transferase activity may be a sensitive biochemical marker for a primitive cell population and may be important in the evaluation of therapeutic effectiveness in acute lymphoblastic leukemia.