To investigate the expressions of cell surface differentiation antigen CD56 and CD11b antigen in acute monocytic leukemic (AML-M(5)) cells and their clinical significance.A total of 113 cases of de nove adult AML-M(5) were examined genetically and immunologically using G-banding technique, interphase fluorescence in situ hybridization (I-FISH) and flow cytometry immunophenotyping, and the results were analyzed in relation to their clinical data.Of the 113 cases, the expression rates of CD56 and CD11b was 28.32% and 73.45%, respectively. The CD56(+) patients had high CD11b expression, and the expression levels of CD11b and CD56 were positively correlated (P0.05).AML-M(5) patients with CD56 positivity and high expression of CD11b often have aberrant karyotypes, commonly involving 11q23/MLL gene abnormality. These patients frequently develop extramedullary infiltration and refractory leukemia often with poor prognosis.