The study was aimed to investigate the expression of HLA class I molecules and MHC class I chain-related molecules A/B (MICA/MICB) in K562 and adriamycin (ADM)-resistant K562 cell lines (K562/AO2) and their effect on cytotoxicity of NK cells. Expression of HLA class I molecules and MICA/MICB on the surface of K562 and K562/AO2 cell lines were analyzed by flow cytometry. Cytotoxicity of NK cells (isolated from 3 healthy persons) against K562 and K562/AO2 cells were detected by LDH releasing assay at different effect-to-target cell ratios (E:T). In blocking experiments, anti-MHC class I monoclonal antibody (McAb) (W6/32, a pan anti-HLA class I antibody) and anti-MHC class I chain-related molecules McAb (BAMO-1, specifically against MICA and MICB) were added to the target cells at E:T of 10:1. The results showed that the expression of MHC class I chain-related molecules on K562 was higher than that on K562/AO2 (P=0.000), and HLA class I molecules were not detectable on both cells. Cytotoxicities of NK cells against K562 and K562/AO2 cells were (29.32 +/- 0.12)%, (45.33 +/- 0.78)%, (58.37 +/- 0.87)%, (72.37 +/- 0.96)% and (12.47 +/- 0.91)%, (24.36 +/- 1.11)%, (33.29 +/- 1.03)%, (53.87 +/- 1.27)% at E:T ratios of 5:1, 10:1, 20:1 and 30:1 respectively (P=0.000), the cytotoxicity of NK cells on K562 cells was significantly higher than that on K562/A02 cells at different E:T ratios. Blocking experiments confirmed that at E:T of 10:1 killing of NK cells against K562 and K562/AO2 cells was efficiently inhibited by BAMO-1, whereas W6/32 had no effect on K562 and K562/AO2 cells. It is concluded that the expression of MHC class I chain-related molecules on K562 and K562/AO2 cells is correlated with NK cell-mediated lysis. NK cells display higher cytotoxicity against parental K562 cells than multi-drug resistant K562/AO2 cells. Down-regulation of MICA/B in multi-drug resistant tumor cell lines leads to reduction of susceptibility to NK lysis.