We have tried to identify carbohydrate structures involved in recognition and/or lysis of K562 target cells by human natural killer (NK) cells. Inhibition studies were performed with mono-, di- and trisaccharides, and with glycopeptides and glycoproteins of known carbohydrate composition. When tested with various monosaccharides, lysis of K562 cells was inhibited only by N-acetylneuraminic acid (NeuAc). Di- and trisaccharides and glycopeptides containing NeuAc or N-glycolylneuraminic acid (NeuGc) all inhibited NK cell-mediated lysis. Among the non-sialylated carbohydrates tested, only Gal beta(1----3)GalNAcol was effective. The inhibitory capacity of sialylated compounds appeared to be dependent on the linkage type of the sialic acid residue; carbohydrates containing alpha(2----6)-linked sialic acids were more potent inhibitors than their alpha(2----3) isomers. Also the sugar to which the sialic acid residue was attached was of importance, NeuAc alpha(2----6)GalNAcol being more effective than NeuAc alpha(2----6)Gal beta 1----R (where R = glucose or oligosaccharide-peptide). Sialylated compounds and free sialic acid had minor or no effects on cell-mediated cytotoxicity by allo-sensitized cytotoxic T lymphocytes. The conjugation of target cells and NK effector cells was not inhibited by carbohydrates that effectively blocked the cytolytic response. These results may indicate that cell-surface carbohydrates containing alpha(2----6)-linked sialic acid are crucial structures in a post-binding event in NK-cell-mediated lysis.