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Experiments in several laboratories have shown that target susceptibility to NK and lymphokine-activated killer (LAK) cytotoxicity is inversely correlated with the target expression of HLA Class I molecules. We present the first direct evidence, obtained by gene transfection, that target cell HLA, A, B expression increases the resistance to the "so-called" non-MHC-restricted cytotoxicity. We have co-transfected, by electroporation, the human beta 2-microglobulin gene and the gene carrying the resistance to geneticin into Daudi cell line. Geneticin selection in culture followed by FACS sorting on the basis of strong positivity with the mAb W6/32 (which is specific for the HLA class I H chain associated to beta 2-microglobulin) have led to the establishment of a HLA+ Daudi cell line permanently expressing HLA A10, A11, and B17 molecules. Studies were performed in vitro to evaluate the susceptibility of these cells to either NK and LAK cytotoxicity. The HLA class I+ Daudi cells exhibit an increased resistance to killing by non-MHC-restricted killer cells (both NK and LAK) as compared with their HLA-Daudi counterpart.