We looked for clonal chromosomal abnormalities in myeloid cell lineages in the bone marrow aspirates from six children with acute lymphoblastic leukemia (ALL). The study was carried out using a combination of MAC (morphology, antibody, chromosomes) and in situ hybridization procedures. In patients whose leukemic cells expressed only lymphoid antigens, we found chromosomal aberrations in CD10- and CD20/22-positive lymphoid cells. Mature CD22+ and CD3+ lymphocytes did not have the chromosomal aberrations. In one patient whose leukemic cells also expressed myeloid-associated antigens, the clonal chromosome aberrations were seen not only in the CD10+ and CD19+ blasts, but also in glycophorin A-positive morphologically nonleukemic erythroblasts.