Abstract： Objective To investigate the effect of interferon-α (IFN-α) on expression of major histocompatibility complex (MHC) class I chain-related protein A (MICA) in the malignant hematopoietic cell lines.Methods The myeloid leukemia cell lines K562 and B-cell lymphoma cell line Raji were treated with IFN-α.The expression of MICA was measured by RT-PCR and immunohistochemical staining at mRNA and protein level.The cytotoxicity of human NK cells to the IFN-α treated malignant hematopoietic cells was detected by MTT method.Results After being treated with 1000 U/ml IFN-α,up-regulation of MICA mRNA in Raji and K562 cells were respectively found in 24 h and 48 hours (t =17.016,P ＜0.05; t =5.616,P ＜0.05).After being treated with IFN-α 72 hours,up-regulation of MICA mRNA in K562 and Raji were found at the dose of 500 U/ml and 1000 U/ml dose (t =6.622,P ＜0.05; t =9.071,P ＜0.05).The mRNA and protein expression of MICA were up-regulated by IFN-o in the two malignant hematopoietic cells in a time and dosedependent manner.After being treated with 1000 U/ml IFN-o for 72 hours,the susceptibility of the two malignant hematopoietic cells to NK cytolysis was significantly increased (t =20.016,P ＜0.05; t =7.969,P ＜0.05).Moreover,the up-regulated susceptibility can be blocked by anti-MICA antibody.Conclusion IFN-α can up-regulate the MICA expression in the malignant hematopoietic cell lines and thereby enhance the susceptibility to cytolysis of NK cells.