NK subsets have activating and inhibitory receptors that bind MHC-I. Ly49A is a mouse inhibitory receptor that binds with high affinity to H2 d in both a cis-and trans-manner. Ly49A cis-associations limit trans-interactions with H2 d-expressing targets as well as mAb binding. We demonstrate that cis-interactions affect mAb effector functions. In vivo administration of anti-Ly49A depleted NK cells in H2 b but not H2 d mice. Despite lack of depletion, in vivo treatment with anti-Ly49A reduced NK killing capabilities and inhibited activation, partially due to its agonistic effect. These data explain the previously described in vivo effects on bone marrow allograft rejection observed with anti-Ly49A treatment in H2 d-haplotype mice. However, prior treatment of mice with poly (I: C) or mouse CMV infection resulted in increased Ly49A expression and Ly49A+ NK cell depletion in H2 d mice. These data indicate that, although Ly49 mAbs can exert similar in vivo effects in mice with different MHC haplotypes, these effects are mediated via different mechanisms of action correlating with Ly49A expression levels and can be altered within the same strain contingent on stimuli. This illustrates the marked diversity of mAb effector functions due to the regulation of the level of expression of target Ags and responses by stimulatory incidents such as infection.