The mechanisms responsible for immunoglobulin G (IgG) immunity against allogeneic platelets are poorly understood. We studied the role that murine recipient CD8⁺ T and natural killer (NK) cells play in immunity against allogeneic platelets. BALB/c mice were depleted of the cells by cell-specific antibodies, transfused weekly with platelets from C57BL/6 mice, and serum IgG antidonor antibodies were measured by flow cytometry. While allogeneic platelet transfusions into wild-type recipients stimulated IgG antidonor antibodies in all mice by the fifth transfusion, CD8-depleted mice had significantly (P < .001) enhanced antibody production. Isotype analysis revealed that CD8⁺ T cells suppressed T-helper 2 (Th2)-associated IgG1 but enhanced Th1-associated IgG2a. Compared with wild-type mice, platelet transfusions into CD8-depleted mice stimulated enhanced intracellular interferon (IFN)-γ production by CD4^- lymphocytes within 24 hours after the first transfusion. The early IFN-γ response correlated with nitric oxide-dependent splenic cytotoxicity (P < .001). In asialo ganglioside monosialic acid 1 (GM1)-depleted mice transfused with allogeneic platelets, the IFN-γ production, splenic cytotoxicity, and IgG antidonor antibody response were significantly suppressed. These results demonstrate that IgG antiplatelet immunity is dependent on an early NK cell-derived IFN-γ response that is negatively regulated by CD8⁺ T cells and suggest that targeting innate NK cell responses may significantly reduce platelet alloimmunization. (Blood. 2004;103:2705-2709)