The Fc receptor for IgG on human natural killer (NK) cells can be identified by a series of murine monoclonal antibodies. When these antibodies with IgG1 isotype (e.g., Leu-11a and Leu-11c), but not with IgM (e.g., Leu-11b and VEP13), were added to NK assay culture against K562 targets of 4-hr analysis, a considerable enhancement of NK activity was induced. An enhancement was shown at a concentration of up to 10(-5) mg/ml of Leu-11a. Furthermore, the present experiments demonstrated that many established cell lines with diverse cell origins expressed the Leu-11 antigens, and enhancement of NK activity by Leu-11 was induced when these Leu-11+ cell lines were used as targets in NK assays. The results that Leu-11a caused an increase in effector-to-target cell conjugates, and that F(ab')2 of Leu-11a was effective in enhancement of NK activity, but the Fab was not, indicated that Leu-11 might become a linkage between the effector and target cells. Significance of this phenomenon was discussed regarding practical application of Leu-11 antibodies in laboratory experiments and in clinical studies.