Natural killer (NK) cell cytotoxicity toward target cells depends on synergistic coactivation by NK cell receptors such as NKG2D and 2B4. How synergy occurs is not known. Synergistic phosphorylation of phospholipase PLC-γ2, Ca²⁺ mobilization, and degranulation triggered by NKG2D and 2B4 coengagement were blocked by Vav1 siRNA knockdown, but enhanced by knockdown of c-Cbl. c-Cbl inhibited Vav1-dependent signals, given that c-Cbl knockdown did not rescue the Vav1 defect. Moreover, c-Cbl knockdown and Vav1 overexpression each circumvented the necessity for synergy because NKG2D or 2B4 alone became sufficient for activation. Thus, synergy requires not strict complementation but, rather, strong Vav1 signals to overcome inhibition by c-Cbl. Inhibition of NK cell cytotoxicity by CD94-NKG2A binding to HLA-E on target cells was dominant over synergistic activation, even after c-Cbl knockdown. Therefore, NK cell activation by synergizing receptors is regulated at the level of Vav1 by a hierarchy of inhibitory mechanisms.