Prior study has revealed that the expression of certain major histocompatibility complex (MHC) class I molecules on the membranes of normal or transformed cells can prevent natural killer (NK)-cell-mediated killing. This has been explained by either (1) the target interference model, or (2) the effector inhibition model. In both cases, MHC class I molecules are concerned but the precise mechanisms are still unclear. The erythroleukaemia K562 cell is known as a NK-sensitive target. This sensitivity has been explained mostly as being due to lack of MHC class I antigens on the K562 membrane. However, several recent studies have indicated that the expression of MHC class I antigens on the cell does not solely explain the protection against NK cells. To elucidate the mechanism of NK-cell-mediated killing, we investigated the killing profiles of the K562 cells by NK cells. Previous studies indicated that the NK-sensitive K562 cells can express some MHC class I antigens on their surface, and become protective against NK-cell killing after treatment with human interferon-gamma (hIFN-gamma). In the present study, we show that this resistance is not rendered by MHC class I expression, because addition of anti-MHC class I monoclonal antibodies (mAb) in the killing assay of hIFN-gamma-treated K562 cells, which express MHC class I antigens as the target, did not restore the sensitivity to NK cells. Moreover, we show that spontaneously occurring K562 clones which could not express MHC class I antigens even after hIFN-gamma treatment, could be protected after treatment with the cytokine. Taken together, these results strongly suggest that the susceptibility of K562 cells to NK cell killing is due to some elements distinct from those related to MHC class I antigens.