The ability of lectins to augment natural killer (NK) cell activity was demonstrated using splenic effector cells from athymic nude mice. The addition of Helix pomatia agglutinin to the assay or pretreatment of the effector cells with lectin resulted in increased cytotoxicity against the human cell line K562, a relatively poor target for spontaneous mouse NK activity. When additional lectins were tested, NK activity was increased markedly by some lectins while others had no effect. The augmentation of cytotoxicity was dependent on the concentration of lectin added. Nylon-wool-nonadherent, nu/nu splenic effector cells mediated the lectin-induced cytotoxicity, and antiasialo GMI plus complement abolished activity, indicating that the cells mediating the cytotoxicity were NK cells and not mature T cells, B cells or macrophages. When spleen cells from mice having different levels of NK activity were evaluated in this system, the magnitude of the lectin-induced cytotoxicity obtained correlated with the level of spontaneous NK activity and no increased cytotoxicity was found using cell populations that had low spontaneous NK activity. By testing a panel of target cells, we found that certain human, mouse and rat cell lines, which had low to moderate susceptibility to spontaneous murine NK activity, were sensitive to lectin-induced NK-cell-mediated cytotoxicity while others were completely resistant; resistance or susceptibility to cytotoxicity was not directly correlated with the ability of lectins to bind to the target cell. The lectin-induced cytotoxicity was independent of interferon (IFN) since detectable levels of IFN were not produced in the cultures of lectins and effector cells, cytotoxicity was not increased when exogenous IFN was added to the assay, and inhibitors of RNA and protein synthesis, which block the action of IFN, had no effect on the cytotoxicity. These results indicate that NK cells can mediate lectin-induced cytotoxicity and the implications of lectin interaction with NK cells are discussed.