Terminal deoxynucleotidyl transferase (TdT) has been cited as a possible biological marker for acute lymphoblastic leukemia (ALL) and the blast phase of chronic myeloid leukemia (CML). In the present study high levels of TdT, comparable to levels present in normal thymus, were observed in circulating leukocytes of a CML patient in blast crisis. Sequential studies revealed disappearance of TdT from circulating leukocytes after remission was induced with vincristine and prednisone therapy. Blood TdT returned to moderately elevated levels in the absence of detectable circulating blasts, 5 months before the reappearance of blast crisis. During blast crisis the malignant cells were devoid of myeloid features on light and electron microscopy and were peroxidase negative and periodic acid-Schiff (PAS) positive. These studies demonstrate that elevated TdT levels may occur with or without detectable morphologic evidence of circulating blast cells. The presence of complement receptors on agranular blasts during the blast phase of chronic myeloid leukemia is a new finding.