The authors examined the expression of myeloid antigens (MyAg): CD11b, CD13, CD14, CD15, and CD33 in 249 adults with lymphoid neoplasms using flow cytometric analysis. In this study, acute leukemia that was myeloperoxidase negative by light microscopy and had at least one lymphoid antigen was defined as acute lymphoblastic leukemia (ALL). The patients were classified as follows: 6 with unclassified ALL, 35 early B precursor ALL, 32 T-ALL, 25 B-cell chronic lymphocytic leukemia (B-CLL) and its variants, 24 B-cell non-Hodgkin's lymphoma (B-NHL), 7 plasma cell disorders, 8 T-CLL, 2 adult T-cell leukemia, and 10 T-NHL. CD11b and CD15 were present in a wide range of lymphoid disorders irrespective of B/T lineage and maturity. Unclassified ALL and phenotypically immature ALL frequently expressed CD13 and CD33, and occasionally expressed CD14. Among early B precursor ALL, CD13, and/or CD33 were significantly associated with the presence of stem cell marker CD34 and the chromosomal abnormality t(9;22). In addition, ALL with deletion of chromosome 7 commonly expressed CD13 and CD33. Taken together, CD13 and/or CD33 positive ALL may originate from a multipotential stem cell. Among mature neoplasms, CD14 was frequently, and CD13 and CD33 were occasionally expressed in B-cell, but not T-cell tumors. These results suggest that CD13, CD14, and CD33 are preferentially expressed in two types of lymphoid neoplasms, namely undifferentiated ALL and mature B-cell lymphoproliferative disorders.