The proliferative activity of bone marrow blasts was determined in 19 patients with acute leukemia during early induction therapy using in vitro tritiated thymidine (3H-TdR) cytoautoradiography and propidium iodide DNA flow cytofluorometry. Following 1-3 causes of treatment, the aliquot of bone marrow blasts in 9 patients, who later achieved remission or marrow aplasia, was reduced to a greater extent that of the remaining 10 patients who failed to respond. In the first group of patients, the 3H-TdR labeling index was increased by 259-653% over the pretreatment value, whereas it was lower than 104% of the pretreatment value in all but one unresponsive patient. Leukemic blast recruitment is conceivable. It might have facilitated remission by increasing the effectiveness of the antileukemic agents administered later.