B-precursor acute lymphoblastic leukemia bone marrow specimens that contained subpopulations of cells with immunophenotypes corresponding to early (CD34) and late (CD20) and (CD22) stages of normal B-cell differentiation were studied. Subpopulations of cells were isolated according to immunophenotype and then analyzed by both a clonogenic assay and molecular genetic methods. Clonal equivalence of the early and late immunophenotypic subpopulations was confirmed for each case by the demonstration of identical lg gene rearrangements. The in vitro colony-forming assay consistently showed a growth advantage for the CD34+ subpopulations over the CD34- subpopulations. CD34 mRNA was detected readily in these isolated precursor cells. When two specimens in which virtually all of the leukemia cells were CD34+ and CD34+CD20+ and CD34+CD22+ subpopulations were also present the CD34 mRNA was limited to the cells without the late-stage differentiation antigens on their surface. Furthermore, the c-myb mRNA was found only in the subpopulations that also contained CD34 mRNA. Our results show that a limited program of differentiation reminiscent of normal B-cell development may be present in this leukemia.