There is a large body of evidence that supports the notion that NK cells exert important immune surveillance functions in vivo, against a variety of virus-infected and neoplastic cells. However, certain targets are not susceptible to lysis by NK cells. The exact mechanism by which resistance or sensitivity is conferred on target cells is not known. We investigated whether the selectivity to NK lysis is a property of the membrane of the target cell. This was examined by the application of a recently developed method which is aimed at changing the membrane structure of the target cell by cell-liposome fusion. Our studies demonstrate that NK-resistant tumor cells acquired sensitivity to lysis by NK cells after fusion with reconstituted vesicles which contained membrane components derived from NK-sensitive target cells. The fusion required the presence of Sendai virus envelope glycoproteins and exogenous lipids (soybean lecithin and cholesterol) for maximal efficiency. This finding was demonstrated in both the human system (with U937 and Raji as NK-sensitive and -resistant cell lines, respectively) and the rat/murine system (with YAC-1 as NK-sensitive target and P815 and YAC-asc as NK-resistant targets). Both the 51Cr-release assay and the single cell assay showed lysis of the modified target cells in a 3-hr incubation period. The magnitude of the cytotoxic activity was found to depend on the concentration of reconstituted vesicles used in the fusion step. The effect seen was specific because target cells were not lysed when fused with vesicles which contained membrane constituents derived from either NK-resistant targets or NK-sensitive targets from another species (human vs mouse). The resistance of modified target cells to lysis by xenogeneic NK cells was not due to failure of membrane fusion, as detected by immunofluorescence, or to failure to form conjugates. These results demonstrate the feasibility of converting a resistant NK target to a sensitive target by cell-liposome fusion. Furthermore, the data indicate that susceptibility to lysis by NK cells is a property of the membrane composition of the target cell. The significance of these findings is discussed.