Objective:To study the expression and function of B7H3 in human myeloid leukemic cell lines,U937 and K562. Methods:The protein and mRNA expression levels of B7H3 were detected by FACS and RT-PCR methods,respectively. The effect of B7H3 in human leukemic cell lines was studied by B7H3 mAb inhibiton assay. Cell proliferation was determined by cell counting. Results:B7H3 expressed in both U937 and K562 cell lines. B7H3 mAb could inhibit cell proliferation of those two cell lines. Conclusion:B7H3 is constitutive expression in human myeloid leukemia cell lines,U937 and K562. B7H3 mAb can conjugate with B7H3 expressed in cell surface,which plays a critical role in the survival and growth of leukemia cells.