Among the rat antibodies tested in this study, monoclonals of IgG2a isotype were the best, IgG1 and IgG2b being also effective, for induction of K-cell-mediated cytotoxicity of relevant nucleated target cells by rat effectors (Nude spleen cells). The single rat monoclonal IgE tested was also active. Rat antibodies of IgM and IgA classes of immunoglobulins were not active in that system. IgG2c and IgD monoclonals were not tested. Using the same panel of reagents, cytotoxicity mediated by human K cells (peripheral blood mononuclear cells) was only induced by the IgG2b class of rat monoclonals.