Perturbations of the generation cycle of human leukemic myeloblasts in vivo by cytosine-arabinoside were studied by in vitro labeling of human bone marrow with tritiated thymidine. Ten patients with myeloblastic acute leukemia at onset of the disease or in relapse received a 4-h infusion of the drug. Mitotic and labeling indexes were calculated before and after chemotherapy. A kinetic pattern indicating a specific action of the drug on the S-phase lasting about 20 h was found. A secondary increase of the labeling index beginning 24 h after treatment was observed. The results suggest a synchronization of the blastic cells in cycle 48 h after cytosine-arabinoside injection.