Anti–protamine-heparin antibodies cause platelet activation and destruction in vivo in a heparin-dependent manner via cross-linking the FcγIIa receptor. Human platelets supplemented with protamine, or protamine-heparin, or protamine-heparin and F(ab)´₂ fragments of the FcRγIIa-inhibiting monoclonal antibody IV.3, or buffer were injected retro-orbitally into NOD/SCID mice. After 30 minutes (baseline), the number of circulating human platelets was estimated (100%), and the IgG fractions of patient sera containing anti–protamine-heparin antibodies (or the IgG fraction from healthy control participants) were injected intraperitoneally. The percentage survival of human platelets was measured after 60, 120, and 300 minutes. Results are shown as a median (range) of experiments performed with anti–protamine-heparin antibodies obtained from 7 patients after CPB (A). Expression of the platelet activation marker human CD62p was estimated at 30, 60, and 120 minutes after platelet injection (B). The control IgG fraction did not cause platelet elimination (A, open circles and dashed line) and minor expression of CD62p (median fluorescence intensity, 40; range, 35-46) at 120 minutes after platelet injection in the presence of protamine-heparin (not shown).