The expression of a cell cycle-related nucleolar protein (p145) antigen was examined in the bone marrow aspirates of 45 individuals, three of whom had no malignant disease; 30 had a diagnosis of acute myeloid leukemia (AML), and 12 suffered from chronic myeloid leukemia (CML). While no evidence of p145 expression was found in the three normal bone marrow samples, it was noted to be the highest in patients with active leukemia, be they AML or blastic crisis of CML. There was a direct correlation between the percentage of blasts and the percentage of p145-positive cells in all patients. Double labeling with tritiated thymidine and p145 in AML patients with active leukemia showed that the majority of S-phase cells contained p145. Myeloblasts in both chronic phase and blastic crisis of CML expressed p145. Nine of 12 AML patients studied during remission had less than 5% p145-positive cells, but three showed 11%, 16%, and 33% positive cells. Since functionally/morphologically, these marrows were normal, the appearance of p145 may indicate a proliferative abnormality preceding maturation arrest and development of relapse. Thus we conclude that p145 is more commonly associated with immature cells and may serve as an early indicator of relapse in AML, but requires further study with larger numbers of patients.