A simplified flow cytometric assay to measure early treatment response in childhood B-lineage ALL. Bone marrow samples from 4 patients were obtained on day 19 of remission induction therapy and stained with anti-CD19, anti-CD10, and anti-CD34 antibodies. Flow cytometric dot plots show CD19 and CD10 expression among mononuclear cells. In patients 1 and 3 (left panels), CD19⁺ coexpressing CD10 (and CD34; not shown) were undetectable, whereas in patients 2 and 4 (right panels) they constituted 0.01% and 0.28%, respectively, of mononuclear cells.